Previous research has shown that low concentrations of thiols will act as radical traps to inhibit autoxidation. When added to a fuel, thiols accelerated the rate of oxygen reaction without a commensurate increase in peroxidation. Evidence for the oxidative addition of thiols to olefins has been found to occur by studying the addition of thiophenol to indene in a model fuel during stressing in the JFTOT apparatus. Two different thiol-indene adducts were found in the effluent, with the product distribution being temperature dependent. This process could account, in part, for the differences in thiol influences on autoxidation observed in model systems and in fuels.