This work reports an electron spin resonance study of the reactions of cysteine, glutathione and penicillamine thiyl radicals with molecular oxygen in frozen aqueous solutions at low temperatures. For all three thiols, the thiyl radical, RS., is found to react with oxygen to form the thiol peroxyl radical, RSOO(.). On the absorption of visible light, RSOO(.) photoisomerizes to the sulfonyl radical, RSO2(.), which subsequently reacts with molecular oxygen to form RSO2OO(.), the sulfonyl peroxyl radical. The identities of the sulfonyl and sulfonyl peroxyl radicals were confirmed by their production by a different route, from sulfinic acid. Sulfinyl radicals, RSO(.), are found as the final radical species in the reactions of thiyl radicals and oxygen. Parallel 17O hyperfine couplings (A parallel) are reported for each sulfoxyl radical and a correlation between the spin density on oxygen and the reactivity of the radical is suggested. As a result of this correlation sulfonyl peroxyl radicals are predicted to be far more reactive than thiol peroxyl radicals. We also report molecular orbital calculations on the nature of the spin density distribution and the molecular geometry of the model radicals CH3SO2(.) and CH3SO2OO(.).