The reduction of chromate ion by l-cysteine in near-neutral aqueous solutions was studied by u.v.–vis spectroscopy. The rate constants for the formation, decomposition and redox degradation of a CrVI thioester intermediate (both in the absence and in the presence of dissolved O2) were determined by the use of a nonlinear least-squares regression to fit the experimental data to a double-exponential integrated rate law. Superoxochromium(III), CrOO2+, could be observed as a long-lived intermediate by the use of u.v. difference spectroscopy (peaks at 238 and 292 nm). The effects of several reaction variables on the yield of the intermediate CrOO2+ were studied. A striking result was that the yield decreased as the concentration of dissolved O2 increased. In the presence of phosphate buffer, the yield showed a maximum at pH 6.8. Formation of CrOO2+ was suppressed by the additives Mn2+, H2O2, d-ribose, 2-deoxy-d-ribose and a high concentration of Ce3+, whereas a low concentration of the latter enhanced its appearance. A novel mechanism for the formation of CrOO2+ from CrVI and l-cysteine (involving the intramolecular formation of an O–O bond) is proposed, and the possible routes for the decay of this intermediate are discussed. This novel mechanism might offer new insights into the redox chemistry of chromate ions.