An accurate fast spectrophotometric method for the determination of cysteine and N-acetylcysteineis presented, based on the oxidation of these amino acids by ferric ions in the presence of ferrozine,whereby a violet-coloured complex is formed which absorbs at 562 nm. The method was satisfactory for thedetermination of cysteine and N-acetylcysteine in samples within the range 0.02-6.00 ggm1-1. Effectsof time, acidity, ferric ions, ferrozine, sodium perchlorate concentrations and the tolerance limit for other amino acids have been reported. The method was applied to the determination of cysteine in amino acids mixtures and N-acetylcysteine in pharmaceutical preparations.