The mechanism of photoinduced electron transfer between sulfur-containing alcohols 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 five hydroxyalkyl sulfides, 2-(methylthio)ethanol (2-MTE), 2,2'-dihydroxydiethyl sulfide (2,2'-DHE), 3-(methylthio)propanol (3-MTP), 3,3'-dihydroxydipropyl sulfide (3,3'-DHP), and 4-(methylthio)butanol (4-MTB), with varying numbers of OH groups and varying locations with respect to the sulfur atom, were determined to be in the range (3.3-4.8) x 10(9) M-1 s(-1) for neutral solutions. The intermediates identified were the CB ketyl radical anion (CB.-), the CB ketyl radical (CBH.), and intermolecularly (S therefore S)-bonded radical cations. An additional absorption band at approximately 400 nm in the transient spectra for some of the hydroxyalkyl sulfides was assigned to the intramolecularly (S-.-O)-bonded species (only for hydroxyalkyl sulfides which could adopt a five-membered ring structure). The spectra of appropriate (S therefore S)(+) and (S-.-O) intermediates for the hydroxyalkyl sulfides were determined from complementary pulse radiolysis studies in acid and neutral aqueous solutions of the hydroxyalkyl sulfides, respectively. The observation of ketyl radical anions and intermolecular (S therefore S)-bonded radical cations of the hydroxyalkyl sulfides was direct evidence for the participation of electron transfer in the mechanism of quenching. Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of formaldehyde formation from the steady-state measurements were determined. The values of these quantum yields indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is a minor photochemical pathway (with a contribution of similar to 5-25% depending on the numbers of OH groups). Competing processes of proton transfer and back electron transfer within the CT complex gave significant contributions to these yields. Detailed mechanisms for the CB-sensitized photooxidation of sulfur-containing alcohols are proposed, discussed, and compared with that for the (OH)-O-.-induced oxidation. One striking feature of the mechanisms is that there is a catalytic influence of neighboring groups on the radical reaction pathways during one-electron oxidation of the hydroxyalkyl sulfides in comparison to comparable reactions of nonsubstituted alkyl sulfides. Support for the mechanisms came in part from an analysis of observed solvent isotope effects on radical quantum yields.