OBJECTIVE: Hair is a crucial aspect of an individual's physical appearance. Thus, there is always a need for technology to care for curls and undulations. In this study, we found that heat treatment with a mixed aqueous solution of cysteamine (2-aminoethanethiol) and fumaric acid was highly effective in improving hair quality. The purpose of this study was to scientifically elucidate the effectiveness of this technology by identifying its active ingredients, evaluating their functionality and analysing changes in the internal structure of hair after treatment. METHOD: We observed changes in hair shape through a hair straightening test, analysed the components using liquid chromatography/mass spectrometry (LC/MS) and investigated the effects on mechanical properties using bending and surface friction measurements. Furthermore, we analysed the hair conditions using small-angle X-ray scattering (SAXS) measurements and evaluated water retention using thermal analysis. RESULTS: In the hair straightening test, even extremely strong curls of African human hair improved. The LC/MS results revealed that the active ingredient was 2-(2-aminoethylthio) succinic acid (ATS). Furthermore, by treating hair with ATS, decreases in the coefficient of friction and bending stiffness and an increase in moisture content were observed. The SAXS measurements revealed that the treatment widened the distance between the intermediate filaments (IFs) inside the hair and improved the orientation of the IFs. CONCLUSION: ATS, produced by the thiol-ene reaction between fumaric acid and cysteamine, acts as an active ingredient in hair shape control. ATS reacted within the matrix and increased the IF-IF distance. This suggests that ATS functions as a crosslinker for keratin proteins. ATS is believed to increase the moisture content of hair and improve hair texture.