Isopentyldiol

What it is

Isopentyldiol is a five-carbon diol with a branched chain, which the Cosmetic Ingredient Review identifies as 3-methyl-1,3-butanediol. The review counts it among the branched diols it describes as very soluble in water, and it has been reported at 97% purity, and above 99% by a cosmetics industry supplier. The EU's CosIng database records a single function for it, solvent, and the review's own ingredient table lists the same. In the review's 2015 industry survey it was used at up to 10% in products applied to the skin and up to 15% in hair conditioners and shampoos; the US Food and Drug Administration's 2017 registrations counted 135 products containing it, 132 of them left on. In this catalogue it is printed on six Korean masks and one German peel-off, and on six of the seven it sits above the point where ingredient order stops indicating quantity.

How it shows up on a label

What it does well

• Keeps other ingredients dissolved in the formula

  Solvent is the only function CosIng records for it, and the Cosmetic Ingredient Review lists the same. This is what the ingredient does in the product, not a claimed effect on skin.

  Strong evidence

• Reviewed as safe as used

  The Cosmetic Ingredient Review's expert panel named isopentyldiol among seven alkane diols that are safe in the present practices of use and concentration, a conclusion published in 2024; three other diols in the same review were left without a conclusion for lack of data. Behind it: an oral LD50 above 5 g/kg in mice, negative results in an Ames test and a second bacterial assay, no sensitisation in a guinea pig maximisation test, no photo-irritation or photosensitisation in guinea pigs, and no eye irritation in rabbits. Australia's industrial chemicals regulator at the time, NICNAS, found it not to be an unacceptable risk to public health in cosmetics at up to 10%.

  Moderate evidence

Worth knowing

• Rated slightly irritating in a small human patch test

  Thirty people had isopentyldiol at an unspecified concentration held against the upper arm under a Finn chamber for 48 hours, alongside 1,3-butanediol and water. Both diols were rated slightly irritating: slight redness was recorded in two people 30 minutes after the chambers came off, and it had gone within 24 hours. Undiluted, it was non-irritating to slightly irritating to rabbit skin. In the guinea pig sensitisation test, the induction doses caused moderate, confluent redness at the injection and application sites, but at challenge there were no reactions and it was classed as non-sensitising. The review weighed all of this and still concluded it is safe as used, which is why the risk level here is low.

Where the EU stands

Sources (6)

Every link goes to the original: a journal, a PubMed record or an EU document.

1. [1]
   Safety Assessment of Alkane Diols as Used in Cosmetics

   International Journal of Toxicology (Cosmetic Ingredient Review) · 2024 · doi:10.1177/10915818231224234

   The published final report, IJT 43(2_suppl):70S-131S. Abstract: the Panel "assessed the safety of 10 alkane diol ingredients", which "are structurally related to each other as small diols, and most are reported to function in cosmetics as solvents", and "concluded that seven alkane diols are safe in cosmetics in the present practices of use and concentration described in this safety assessment, but that the available data are insufficient to make a determination of safety for three ingredients, namely 1,4-Butanediol, 2,3-Butanediol, and Octanediol." Isopentyldiol is not among the three. Checked 10 October 2026.

2. [2]
   Post-Meeting Announcement, 145th Meeting of the CIR Expert Panel, December 2017

   Cosmetic Ingredient Review · 2017

   Alkane Diols, 10 ingredients, split conclusion: "the following 7 (of 10 total) alkane diols are safe in cosmetics in the present practices of use and concentration described in the safety assessment", listing Butyl Ethyl Propanediol, 1,10-Decanediol, Hexanediol, Isopentyldiol, Methylpropanediol, 1,5-Pentanediol and Propanediol. Checked 10 October 2026.

3. [3]
   Safety Assessment of Alkane Diols as Used in Cosmetics - tentative report for public comment

   Cosmetic Ingredient Review · 2017

   The April 2017 tentative report, the source for the data this entry quotes; the conclusion above supersedes its wording. Isopentyldiol (CAS 2568-33-4) has function "Solvent" in Table 1 and "has been reported to be 97% pure", and "> 99% pure as reported by a cosmetics industry supplier". 135 reported uses in 2017, 132 leave-on; 2015 maximum concentrations "15% Isopentyldiol (in hair conditioners, non-coloring shampoo, and other hair preparations, non-coloring)" and 0.33-10% for dermal contact. "In a NICNAS report, Isopentyldiol was determined not to be an unacceptable risk to public health in cosmetic products up to 10%." Oral LD50 "> 5 g/kg Isopentyldiol in mice"; negative "in an Ames test (up to 10,000 µg/plate) and in a liquid suspension assay". Undiluted it "was non-to-slightly irritating to rabbit skin"; "Isopentyldiol (concentration not specified) and 1,3-Butanediol (concentration not specified) were slightly irritating in a 48-hour Finn chamber skin test" on 13 men and 17 women, with slight erythema in two subjects 30 minutes after removal that "resolved within 24 h". In the guinea pig maximisation test it "was non-sensitizing. However, during intradermal injection at induction and topical induction, moderate and confluent erythema were observed." Undiluted, "neither a photo-irritant nor a photo-sensitizer", and non-irritating to rabbit eyes. "The branched alkane diols among these ingredients are very soluble in water". Checked 10 October 2026.

4. [4]
   Quick Reference Table: CIR conclusions by ingredient

   Cosmetic Ingredient Review · 2024

   Lists Isopentyldiol with conclusion S (safe), citing IJT 43(Suppl. 2):70S-131S, 2024 - the same citation the table gives its alkane diol neighbours such as 1,10-Decanediol. Checked 10 October 2026.

5. [5]
   CosIng entry: Isopentyldiol

   European Commission, CosIng database

   INCI name ISOPENTYLDIOL, status active, CAS 50468-22-9 / 2568-33-4. Function: solvent. No annex number, cosmetic restriction or maximum concentration is recorded. The chemical-name field reads "3-Methylbutane-1,2-diol", which matches the first CAS number; the Cosmetic Ingredient Review identifies the ingredient by the second, 2568-33-4, as 3-methyl-1,3-butanediol. Checked 10 October 2026.

6. [6]
   Regulation (EC) No 1223/2009 on cosmetic products, consolidated text of 18 May 2026

   Official Journal of the European Union · 2026

   A sweep of the full consolidated text finds no occurrence of isopentyldiol, methylbutanediol in either numbering, or either CAS number 2568-33-4 or 50468-22-9. The two methylbutane entries in Annex II are unrelated substances, 2,3-dichloro-2-methylbutane and (R,S)-2-amino-3,3-dimethylbutane amide. Checked 10 October 2026.