The thiol moiety is prone to oxidative free radical formation, which may be important in mediating the toxicity of some thiol-containing compounds. The oxidation of the compounds cysteine, cysteamine, N-acetylcysteine, glutathione, penicillamine, and captopril were studied using ESR and oxygen uptake techniques. Lactoperoxidase, with hydrogen peroxide to provide oxidizing equivalents, was used to initiate the oxidation. The reaction appears to be strongly peroxide dependent, with either exogenous H2O2 or thiol-derived peroxide driving the reaction.