The mechanism of photoinduced electron transfer between sulfur-containing carboxylic acids and the 4-carboxybenzophenone (CB) triplet state in aqueous solution was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by six sulfur-containing acids, with varying numbers of COO- groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3-2.1) X 10(9) M(-1) s(-1) depending on the charge of the acid molecule. The observation of ketyl radical anions and intermolecular (S therefore S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanism of quenching. An additional absorption band at approximately 410 nm in the transient absorption spectra for some of the acids was assigned to intramolecularly (S therefore O)-bonded species (for acids for which a five-member ring structure was sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of CO2 formation and CB disappearance from the steady-state measurements were determined. The values of these quantum yields clearly indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is the main photochemical pathway (contribution of similar to 90%). Competing processes of proton transfer and back electron transfer within the CT complex gave only minor contributions to these yields. A detailed mechanism of the CB-sensitized photooxidation of sulfur-containing carboxylic acids is proposed, discussed, and compared with that for sulfur-containing amino acids in aqueous solution.