The anaerobic oxidation of cysteine (H₂L) to cystine by iron(III) has been followed over the pH range 8.2–11.6 by use of a stopped-flow high-speed spectrophotometric method (the results obtained below pH 8.6, however, were not reproducible and no attempt was made to establish a rate law over this range). Between pH 8.6 and 11.2 the experimental data were accurately reproduced by assuming that the bis-cysteine complex, [Fe(OH)L₂]^2–, reacts with the mono-cysteine complex, [Fe(OH)L], to yield two iron(II) ions, one cystine, and an unoxidised cysteine with a second-order rate constant of 8.36 × 10³ dm³ mol^–1 s^–1. The predominant complex species present in solution is the purple [Fe(OH)L₂]^2– which exhibits an absorption maximum at 545 nm (shoulder at 445 nm) and has a molar absorption coefficient of 3 175 dm³ mol^–1 cm^–1. The other complex present is also purple and is [Fe(OH)L]. The absorption spectrum of this species (obtained over the pH range 5.5–8.0) exhibits a maximum at 503 nm (shoulder at 575 nm) and has a molar absorption coefficient of 1 640 dm³ mol^–1cm^–1. The kinetic results were also used to calculate values of the first and second protonation constants for cysteine (log K₁^H= 10.34, log K₂^H= 8.32) which compare very favourably with previously published values (obtained by potentiometric titration). Finally, a value of log K₂^M= 4.76 for the reaction [Fe(OH)L]+ L^2–⇌[Fe(OH)L₂]^2– was also extracted from the kinetic data. All measurements were carried out at 25 °C in solution of ionic strength 0.10 mol dm^–3(KCl).