What it is
The EU's CosIng database gives its chemical name as docosyltrimethylammonium methyl sulphate, and lists three functions: antistatic, hair conditioning and cleansing surfactant.1 In plain terms, a behenyl chain of 22 carbon atoms is attached to a positively charged trimethylammonium group, and the methosulfate is the negatively charged ion that balances it.2
That third function is why "Cleanses and foams" comes first among the roles further down this page: they are taken from the EU database, which records every listed function without saying which one matters. Cleansing is not what it is used for in hair products; researchers study it as one of the common cationic surfactants in conditioners.2
What it does in hair
Cationic surfactants are considered the backbone of a conditioner. Their positive charge is drawn to the negatively charged surface of hair, so they deposit there, especially on weathered or damaged areas, reduce static and make combing easier.3 The conditioner ingredients guide shows how they work with the rest of a formula.
A 2026 laboratory study compared behentrimonium methosulfate with three other cationic surfactants, each at 1% in a simple conditioner. All four conditioned both healthy and bleached hair. Its 22-carbon chain helped it deposit and cut wet combing force more than the shorter-chain Cetrimonium Chloride, but Behentrimonium Chloride did better still. As the two behentrimonium compounds differ only in their counter-ion, the authors propose that the larger methosulfate ion separates less readily and makes the ingredient more water-soluble, which helps a formula stay stable but may limit how much ends up on hair. None of the conditioners reversed the oxidative damage in bleached hair.2
Why it is not a sulfate cleanser
Sulfate cleansers such as Sodium Laureth Sulfate are anionic surfactants, with a negative charge.4 Behentrimonium methosulfate is cationic, and the sulfate in its name belongs to the methyl sulfate ion that balances its positive charge.12 This site's Sulfates group, which marks foaming sulfate cleansers, never includes it. What sulfate-free means lists other names with sulfate in them that are not cleansers.
What official sources say
Behentrimonium methosulfate has no entry in the annexes of the EU cosmetics regulation. Its chloride counterpart is regulated twice, as a conditioner in Annex III, entry 287, and as a preservative in Annex V, entry 44, which covers alkyl (C12-22) trimethyl ammonium bromides and chlorides; neither entry names the methosulfate.5 The EU limits on behentrimonium chloride are therefore not limits on this ingredient. CosIng links an opinion of the EU's Scientific Committee on Consumer Safety to the chloride, and none to the methosulfate.61
How it appears on a label
As Behentrimonium Methosulfate, its name in the EU glossary, where it sits two entries after Behentrimonium Chloride.7 Rinse-out conditioners are built mainly on cationic surfactants and long-chain fatty alcohols4, so expect it near names such as Cetearyl Alcohol. A label gives no concentrations, so it cannot show how much is present. The hair product scanner shows the role of every name on a list.