Isostearyl Isostearate

Isostearyl isostearate is rated Clogging, on a comedogenic 4 of 5 that is real and is this exact ester's — and that rests on one 1989 rabbit-ear assay, with no second reading we could find. The esters it is constantly confused with on a label — isopropyl isostearate, propylene glycol isostearate — are different molecules with their own scores.

What it is

Isostearyl isostearate is an emollient ester: isostearic acid esterified with isostearyl alcohol, both branched eighteen-carbon chains. On a label it is Isostearyl Isostearate; its CAS number is 41669-30-1. A formulator reaches for it for slip and a rich, cushioned, non-greasy feel. It is not one pure compound, and that matters when you read a number attached to the name. The USP-NF monograph defines it as the esterification product of isostearic acid — itself "a mixture mainly of saturated branched 18 carbon-chain (C18) fatty acids, linear hexadecanoic (C16:0) and octadecanoic acids (C18:0)" — with isostearyl alcohol, and sets the assay at not less than 85.0% isostearyl isostearate. So the commercial ingredient is a mixture of predominantly C36 branched esters, and the single structure drawn against CAS 41669-30-1 is a representative constituent, not the whole material. The "iso" in both halves of the name is methyl branching, which is what distinguishes them from straight-chain stearic acid and stearyl alcohol; we note that as chemistry, not as a claim about pores. One thing is worth stating plainly at the top, because it is the reason this page is separate. "Isostearyl isostearate" is not "isopropyl isostearate," and neither is "propylene glycol isostearate." All three are isostearate esters, they read almost identically in an ingredient list, and they sit side by side in comedogenic charts — but they are three different materials, made with three different alcohols, and the legacy literature scored them separately. The number on this page belongs to the first one only.

Why we rate it Clogging

We rate it Clogging at Low confidence, because the evidence is the legacy rabbit-ear assay and nothing stronger. What this page exists to add is exactly how thin, and how narrow, that single number is. Where the number comes from. James Fulton's 1989 survey in the Journal of the Society of Cosmetic Chemists is the source of nearly every comedogenic score in circulation, and we read its Table I off a render of the journal scan rather than a list quoting it. Among the esters it prints a row for isostearyl isostearate: comedogenicity grade 4, on a 0-to-5 scale. The row carries no asterisk, and it sits in the ester section rather than the oils section — so Fulton's oils footnote, "results depend on source of raw material," does not attach to it. On his own published key, "a minimal grade of 0 to 1 is not considered significant," 2 to 3 is "borderline," and only "a grade of 4 to 5 is uniformally reproduceable and considered positive." So the 4 sits in the band he treated as a real finding rather than noise, and it is a graded reading — not the scale's calibration point, which is isopropyl myristate at 5. Now the part the charts leave out. This 4 has no second reading we could find, and the material that was tested twice is not this one. Morris and Kwan, in 1983 — the independent rabbit-ear survey on the same 0-to-5 scale that we hold, run in a different laboratory six years before Fulton — scored isopropyl isostearate 4 and propylene glycol isostearate 3 to 4. They did not test isostearyl isostearate. Fulton's own 1984 survey, which we have now read in full, corroborates a number of his 1989 esters, but isostearyl isostearate is not among the ones it repeats. So where isopropyl palmitate has two labs and isopropyl myristate has a whole literature, isostearyl isostearate has one row, in one paper, from one set of three rabbits in 1989. The rating is Clogging because that row is a positive result on Fulton's own key and we have no reason to overturn it; the confidence is Low because a single uncorroborated rabbit-ear reading is exactly what Low confidence is for.

What the evidence doesn’t tell you

Everything above rests on one rabbit-ear reading. Fulton's method: the ingredient diluted to roughly 10% in propylene glycol, about 1 mL applied to the inner ear of three New Zealand albino rabbits, five days a week for two weeks, follicles then scored for keratosis under a micrometer. That is not a human face, not a finished product, and not a concentration anyone wears — and Fulton himself called his survey "not at all definitive but simply designed to stimulate research," and warned that the rabbit ear is more sensitive than human skin, so that "not everything that irritates this model will also irritate human skin." The single most important limit is corroboration, and there is none. We identified no comedogenicity assay of isostearyl isostearate outside Fulton 1989: not in Morris and Kwan 1983, not in Fulton's own 1984 paper, and not in Nguyen, Dang and Maibach's 2007 rabbit assay, whose fourteen test materials include several esters from this family and, in the abstract we have read, not this one. The related isostearates people line up beside it are different molecules with their own records — isopropyl isostearate has four legacy readings from three research groups, propylene glycol isostearate has one that we can find — and a score for a neighbour is not a score for this ester; conflating them is the exact error this site exists to catch. So the 4 is a real reading, correctly attached to the right material, and also a reading of one, with nothing standing beside it. There is a second limit inside the row itself. Fulton printed a name, not a specification: no supplier, no trade grade, no purity, no isomer profile. Since the ingredient is a mixture of branched esters rather than one compound — the USP-NF assay allows up to 15% of the material to be something other than isostearyl isostearate — we cannot say that what he tested in 1989 is compositionally what is in a tube now. That is not a reason to discount the 4, and it is not licence to read it as a measurement of any particular modern grade. There is human data on this ester, and none of it counted a comedone. We identified no human comedogenicity study of isostearyl isostearate. What exists, summarised in the Cosmetic Ingredient Review's assessment, measured other endpoints: deuterated material applied to forearms for three hours was found mostly at the surface of the stratum corneum, and a plastic occlusion stress test found it improved the barrier's water permeability. Where a material sits after three hours, and what it does to water loss, are not measures of follicular plugging, and we do not read them either way. We are not going to tell you why it clogs. The tempting move is a mechanism — that a large branched ester spreads into the follicle and lingers — and the content mills state it as fact. No comedogenicity paper we read demonstrates a mechanism for this material; the assay counted comedones, it did not explain them, and a mechanism asserted to dress up a rating is not evidence. Concentration is doing work the bare number hides. The assay tested at about 10%, and a comedogenic grade is a property of a material at a dose, not a fixed trait it carries into a low-percentage formula. But Fulton distrusted that reassurance in print, writing of the major offenders his paper names — "we are not convinced of the statement that lower concentrations of these compounds can be safely used with no comedogenic consequences" — so it points both ways at once: against assuming a small percentage is automatically safe, while conceding that the safe threshold was never established. The Cosmetic Ingredient Review's safety assessment of the alkyl esters judged the class, including isostearyl isostearate, safe as used when formulated to be non-irritating; that is a toxicity-and-irritation judgement, not a comedogenicity finding, and the two questions are separate. And the assay itself has been contested for decades: Draelos and DiNardo, in 2006, tested finished products on human backs and found that isolated ingredient scores did not transfer to the formulas they ended up in, and a 2025 review in JAAD Reviews found that no standardised comedogenicity test exists and that "non-comedogenic" is an unregulated label. Those cautions apply to a positive grade as much as to a reassuring one.

Where you’ll see it

On a label it appears as Isostearyl Isostearate. It is a structuring emollient most at home in anhydrous colour cosmetics — lipstick, cream and stick foundation, balms — where its slip and cushioned feel are wanted, and it also turns up in moisturisers. We have not audited its market share, and label position is not a dose, so treat that as the categories it suits rather than a measured prevalence. The practical point is again the neighbours. "Isostearyl Isostearate," "Isopropyl Isostearate" and "Propylene Glycol Isostearate" are three different esters that read almost identically on a label and are scored separately in the legacy literature. If you are checking a product, check which one is actually printed.

Sources

Each source says what it actually is. Several widely-cited “sources” for comedogenic ratings are republishing the same 1989 assay, and counting them as independent agreement is how a thin evidence base gets made to look thick.

1. Fulton JE. "Comedogenicity and irritancy of commonly used ingredients in skin care products." J Soc Cosmet Chem 40:321-333 (1989) The primary source of nearly every comedogenic number in circulation, read off a render of the journal scan rather than a list quoting it. Method: ingredient at roughly 10% in propylene glycol, 1 mL to the inner ear of three New Zealand albino rabbits, five days a week for two weeks, follicular keratosis scored under a micrometer — three animals, a rabbit ear, not a human face. Its Table I prints isostearyl isostearate at a comedogenicity grade of 4 on a 0-to-5 scale, in the ester section, with no asterisk (so the oils footnote "results depend on source of raw material" does not apply). Fulton's own key: 0 to 1 "not considered significant," 2 to 3 "borderline," 4 to 5 "considered positive." Fulton names the material and nothing else about it: no supplier, grade, purity or isomer profile is reported for any row. This is the only document we found that assigns isostearyl isostearate a comedogenicity score. The 4 that consumer ingredient checkers print is the same number as this row; we have not traced which of them read the paper and which copied one another, and matching numbers do not by themselves establish a chain.
2. Morris WE, Kwan SC. "Use of the rabbit ear model in evaluating the comedogenic potential of cosmetic ingredients." J Soc Cosmet Chem 34:215-225 (1983) An independent primary rabbit-ear assay on the same 0-to-5 scale, six years before Fulton and in a different laboratory — cited here for what it does NOT contain. It scored isopropyl isostearate 4 and propylene glycol isostearate 3 to 4, but it did not test isostearyl isostearate at all. It is the reason we keep the three isostearate esters separate rather than pooling their scores. Method: rabbit ear canal, three rabbits, applied undiluted unless otherwise stated, fourteen applications, microscopic follicular grading. Morris and Kwan are not a Fulton republisher — on other materials they contradict him in print, scoring sodium lauryl sulfate 0 and isocetyl stearate 0 to 1.
3. Nguyen SH, Dang TP, Maibach HI. "Comedogenicity in rabbit: some cosmetic ingredients/vehicles." Cutan Ocul Toxicol 26:287-292 (2007) A later rabbit-ear assay from a group unconnected to Fulton, on its own 0-to-4 scale — a different scale from the 0-to-5 papers, so nothing in it could be merged with Fulton's number anyway. Cited here, like Morris and Kwan, for what it does not contain. We have read its published abstract and not its full text; the abstract lists fourteen test materials in order, among them isopropyl isostearate, isocetyl stearate and myristyl myristate, and isostearyl isostearate is not one of them. It is the most recent primary assay we hold, and it did not test this ester.
4. Fiume MM, Heldreth BA, Bergfeld WF, et al. (Cosmetic Ingredient Review Expert Panel). "Safety Assessment of Alkyl Esters as Used in Cosmetics." Int J Toxicol 34(2 Suppl):5S-69S (2015) An expert-panel safety review of the alkyl esters, isostearyl isostearate among them. The document we read is the Final Amended Report — status line "Final Amended Report, Release Date: April 12, 2013" — published in Int J Toxicol in 2015, not one of CIR's tentative or comment-stage drafts of the same assessment. Cited for narrow things: the panel concluded the alkyl esters are "safe in the present practices of use and concentration described in this safety assessment when formulated to be non-irritating," and it collects the human work on this ester — deuterated material on forearms for three hours found mostly at the stratum corneum surface, a plastic occlusion stress test showing improved water permeability barrier function, and an increase in the penetration rate of 5-fluorouracil through excised human skin while the ester itself was judged not a penetration enhancer. A safety assessment weighs toxicity, irritation, penetration and sensitisation — not follicular plugging — and the report contains no comedogenicity discussion for this ingredient, so it is neither evidence for nor against the rating and we do not use it as either.
5. Draelos ZD, DiNardo JC. "A re-evaluation of the comedogenicity concept." J Am Acad Dermatol 54:507-512 (2006) A "Current Issues and Opinion" piece in JAAD that tested finished products on human upper backs and found products testing non-comedogenic despite containing ingredients the rabbit assay had scored comedogenic. Cited because it cuts against a high rabbit-ear grade as much as a low one: it is evidence that an isolated ingredient's score does not transfer intact to the formula it ends up in. It is not a same-substance rabbit-versus-human validation, and we do not present it as one.
6. "Comedogenicity in cosmeceuticals: a review of clinical relevance, regulatory gaps, and future directions." JAAD Reviews (2025) A recent peer-reviewed review of the whole field, covering literature from 1972 onward, and not authored by Fulton or drawn from the consumer lists that print his numbers. Cited for the field-wide caveat that no standardised comedogenicity test exists and that "non-comedogenic" is an unregulated label — the caution that governs every number on this page.
7. United States Pharmacopeia — National Formulary, monograph "Isostearyl Isostearate" (USP-NF M7021) A compendial identity and purity standard, cited for material identity only. It defines the ingredient as "obtained by esterification of isostearic acid which is a mixture mainly of saturated branched 18 carbon-chain (C18) fatty acids, linear hexadecanoic (C16:0) and octadecanoic acids (C18:0), with isostearyl alcohol," and sets the assay at not less than 85.0% isostearyl isostearate. That is the document behind the statement that this is a mixture rather than one pure molecule. A monograph specifies what a material must contain; it tests nothing about skin, and we draw no comedogenic conclusion from it.
8. SpecialChem — Cosmetics Ingredients (INCI) directory, "Isostearyl Isostearate" A supplier-facing INCI reference database, not a comedogenicity source. Cited only for material identity and formulation role: CAS 41669-30-1, the ester of isostearyl alcohol and isostearic acid, and its declared functions of emollient, binding agent and skin conditioning. Trade and supplier databases describe what a material is and what it is sold to do; they do not test pores, and we do not read a comedogenic verdict out of one.

Others in the same family

Esters behave similarly enough that the evidence for one is often wrongly read across to the others. These are its structural relatives, not a guess. What the evidence says about esters as a family ›

• Butyl Stearate Clogging Low
• Caprylic/Capric Triglyceride Clear Low
• Cetyl Acetate Clogging Low
• Cetyl Palmitate Clear Low
• Decyl Oleate Clogging Low
• Isocetyl Stearate Caution Low
• Isopropyl Isostearate Clogging Low
• Isopropyl Linoleate Clogging Low

Last reviewed 2026-08-01 · How we decide

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