The associate formed by triiodide ion and hexadecylpyridinium chloride (cetylpyridinium chloride; CPC) micelles was used to enhance the kinetic spectrophotometric determination of cysteine and cystine by their inductor effect on the iodine-azide reaction. The reaction rate was monitored at 500 nm, the maximum absorption wavelength of the triiodide-CPC associate. The micellar medium allowed the determination of cysteine and cystine at concentrations between 1 and 15 ng ml - 1 and 0.2 and 1.3 μg ml - 1 , respectively, the detection limits (3σ) for the two inductors being 0.6 and 0.12 μg ml - 1 , respectively. Cysteine and cystine were simultaneously determined by measuring the absorbance at two fixed times, 3 min and 1 h. Cysteine: cystine ratios between 1:50 and 1:300 were determined with relative standard deviations of 1.2% for cysteine and 2.3% for cystine. The proposed method was applied for the determination of cysteine in pharmaceuticals.