Maleic acid

What it is

• The cis form of butenedioic acid. Its carbon-carbon double bond next to two acid groups makes it a reactive Michael acceptor that can bond with thiol (-SH) groups. Di Foggia 2021
• In cosmetics it has been used as a fragrance ingredient and pH adjuster at low concentrations. CIR 2007
• Olaplex No. 1 contains its diamine "formulated in water with maleic acid". Gargano 2018

How it works

• Claimed: it reacts with free thiols in damaged keratin to form carbon-sulfur bonds, re-linking broken cysteines.
• Where this chemistry clearly works is reduced hair. After a perm or other reducing treatment, hair has many free thiols. Researchers have used maleic-acid derivatives to attach to these. A maleic-acid and cystine amide gave repeatable perms with higher disulfide retention than conventional oxidative perming (85% versus 72%). Liu 2026 A maleic-modified amino silicone grafted onto reduced hair made the surface highly water-repellent and cut wet combing work by 87%. Liu 2026 A maleate monoester surfactant grafted to reduced hair lowered dry combing work by 76%. Liu 2026
• Where it is doubtful is bleached hair. Bleaching converts cystine mostly to cysteic acid, not thiols. An independent spectroscopy study of maleate-based restorers on bleached hair found no rise in cortical disulfides and no direct evidence of thiol-maleate cross-links, though surfaces looked smoother. Di Foggia 2021; Grosvenor 2018
• Its acidity also lowers product pH, which by itself smooths damaged hair (see acidic rinses and pH).

What the studies show

• No cortical disulfide increase and no direct evidence of cross-linking after maleate treatment of bleached hair (independent), but better cuticle layering on electron microscopy. Di Foggia 2021
• On reduced hair, a maleic-acid perming agent matched oxidative perming with less colour change and cuticle damage. Liu 2026
• On reduced hair, a maleic-modified silicone built a highly hydrophobic surface, lowering bound water from 4.46% to 2.81% and raising tensile strength by 9.1%. Liu 2026
• We found no peer-reviewed controlled study of a maleic-acid bond builder used with bleach on human hair.

How it's used in studies

• Patent examples use maleate salts around 3%. A competitor patent estimates that bleach-additive use would be higher, most likely above 5%.

Side effects and safety

• Irritant at high strength. In the safety review, 20% maleic acid applied daily to the forearm for 6 weeks caused acute blistering dermatitis in 17 of 50 people, and 10% at pH 1 caused permanent corneal opacity in rabbits. It was not mutagenic in the standard bacterial test. CIR 2007
• The panel judged it safe only as a pH adjuster, reasoning that most of it is neutralised to maleate salts in products. That review did not cover leave-on bond-builder concentrations. CIR 2007
• Large doses damage kidney tubules in animals, which is not expected from hair use. CIR 2007
• It is a permitted cosmetic ingredient in the US, EU and UK.

Bottom line

It suits people looking for a cheaper alternative to branded bond builders after bleaching. Expect conditioning-level benefits, because covalent repair is unproven. A citric-acid conditioner is a comparable low-cost option with manufacturer data. Zhang 2025 We rate the evidence D: strongest on reduced hair, weakest on bleached hair. The safe leave-on concentration on scalp hair has not been formally assessed. See bond builders.