We have characterized and quantified pathways by which sulfide radical cation complexes of N-methionyl peptides (Met-Met, Met-Met-Ala, and Met-Leu) transform into various products through the reaction with superoxide or oxygen. Sulfide radical cations were generated photolytically by the reaction of the peptides with either triplet carboxybenzophenone ((CB)-C-3) or hydroxyl radicals (HO.). Sulfide radical cations of Met-Met and Met-Met-Ala, generated through (CB)-C-3, formed intramolecularly sulfur-sulfur three-electron-bonded radical cation complexes, [R2S therefore SR2](+), which efficiently reacted with superoxide to yield the respective disulfoxides Met(O)-Met(O) and Met(O)-Met(O)-Ala. Competitively, monomeric sulfide radical cations and [R2S therefore SR2](+) converted intramolecularly into sulfur-nitrogen three-electron-bonded complexes, [R2S therefore N(R)H-2](+), which reacted with both superoxide and molecular oxygen to yield azasulfonium derivatives. Among these azasulfonium derivatives the C(S),S(R) diastereomers (AS LI) were generally formed in about 1.5-3.8-fold excess over the C(S),S(S) diastereomers (AS I), indicating some diastereoselectivity in the reaction mechanism. Representative quantum mechanical calculations for the azasulfonium diastereomers of L-Met showed that the energy difference between both diastereomers was small, 1.9 kcal/mol (electronic energy) or 1.3 kcal/mol (gas-phase free energy). In complementary experiments, complex [R2S therefore N(R)H-2](+) was generated through the reaction of the peptides with HO.. Hen, the azasulfonium diastereomers were generated predominantly by the reaction of [R2S therefore N(R)H-2](+) with molecular oxygen. The diastereomeric ratios [AS II]/[AS I] were generally higher when the azasulfonium products were formed via the reaction of [R2S therefore N(R)H-2](+) with superoxide instead of with molecular oxygen. The reaction of superoxide with the sulfur radical cation complexes most likely proceeded via an inner-sphere mechanism, i.e. radical-radical combination where the addition of superoxide to [R2S therefore SR2](+) yielded an intermediary persulfoxide, R2S(+)-O-O(-), and the addition of superoxide to [R2S therefore N(R)H-2](+) gave an intermediary hydroperoxysulfurane, R(H)N-S(R-2)OOH.