The stoichiometry of the copper catalysed alkaline autoxidation of cysteine has been investigated. Products of this oxidation reaction are cystine, H{2}O{2} and H{2}O. No oxygen containing sulfur acids are produced as long as cysteine is present in the reaction liquid.The H{2}O{2} generated reacts with cysteine selectively to form cystine and H{2}O with r{H}{2}{O}{2} = k{H}{2}{O}{2} [CyS][H{2}O{2}] (k{H}{2}{O}{2} = 0.17 1 mol}-{}1{ s}-{}1{).The reaction between H{2}O{2} and cysteine is not catalysed by the copper complex as present during the catalytic autoxidation reaction. On the other hand, experiments carried out in the absence of oxygen show a marked catalytic effect ascribed to the presence of a Cu(I) complex.Accumulation curves of H{2}O{2} have been measured during the copper catalysed autoxidation of cysteine. Information about the rate of production of H{2}O{2} at the catalytic site has been obtained by making use of these accumulation curves together with the kinetic data obtained for the reaction of H{2}O{2} with cysteine. It is concluded that apart from a two electron reduction of dioxygen to H{2}O{2} a four electron reduction to H{2}O should also be taken into account.The selectivity of the oxidation reaction justified a reconsideration of the free thiyl radical mechanism proposed in the literature to occur during the copper catalysed autoxidation of cysteine