The anaerobic oxidation of cysteine, Cys, by Mn(III) in acetic acid solutions has been followed by use of a stopped-flow spectrophotometric method at a temperature of 20 o C. The formation and disappearance of the [Mn(OAc) 2 Cys] - complex was monitored at 350 nm. The rate depends strongly on the acetic acid concentration (and hence also on pH) and led to the conclusion that more than one cysteine-containing species was involved. These mono-cysteinyl complexes are formed by the loss of two protons from the cysteine - one from the - SH and the other from either the -NH 3 + or, more likely, the -COOH which is partially protonated at the low pH values involved (0.5-2.5). The rate-determining reprotonation of the bound -COO - (or -NH 2 ) is then accompanied by internal electron transfer yielding Mn(II) and the cysteinyl radical, Cys*, which then dimerises to form (inactive) cystine. At high acetic acid concentrations (60-90% AcOH) the tris-acetato species, [Mn(OAc) 3 ], predominates together with some of the bis-complex, [Mn(OAc) 2 ] + , and the active species is [Mn(OAc) 2 Cys] - which decomposes with a rate constant of k 2 =16.8+/-0.9 M - 1 s - 1 . At low acetic acid concentrations (20-30% AcOH) the mono-acetato species predominates and the reactive species is [Mn(OH)Cys] for which the rate of decomposition=k 2 ' =(1.32+/-0.11)x10 4 M - 1 s - 1 . The relative values of the rate constants obtained are discussed, as is the bonding of cysteine to manganese(III).