A new, effective, specific quaternary ammonium compound, N-dodecyl-amino-betaine-2-mercaptoethylamine hydrochloride (dabm), is synthesized to confer permanent characteristics (antimicrobial properties, improved dyeing and softening) on keratin material (wool, human hair). We have studied the reaction between bisulphite-treated wool and dabm as a function of time in a buffered medium, and monitor it with ftir/atr spectroscopy, which allows us to follow the changes in cysteine-S-sulphonate groups (at 1023 cm–1) located in the cuticula of keratin fibers. We believe dabm links permanently to keratin fibers, given the disappearance of the cysteine-S-sulphonate band during treatment. Trend approximation and corresponding functions are obtained for experimental data.