Sulfur-containing amino acids and alanine were oxidized via photosensitization by 4-carboxybenzophenone (CB) in alkaline aqueous solutions. The mechanism of this reaction was examined using steady-state and laser flash photolysis techniques. The rate constants were determined for the quenching of the CB triplet state by five sulfur-containing amino acids and alanine and were found to be similar to 10(9) and 1.8 x 10(8) M(-l) S-1, respectively. The observation of the (S therefore S)(+) radical cations of some of the amino acids showed that the quenching process involves an electron transfer from the sulfur atom to the triplet state of CB. A slow process of formation of the ketyl radical anion occurring on the microsecond time scale was assigned to the one-electron reduction of CB by the alpha-aminoalkyl radicals that were formed earlier as a result of an intramolecular electron transfer from the carboxyl group to the sulfur-centered radical cation followed by decarboxylation. For thiaproline, the pseudo-first-order rate constant, k’(obs) which characterizes the slow process of secondary ketyl radical anion formation, is linearly dependent on the CB concentration over the pH region 9.4-13.4. The slope of such a plot gives the bimolecular rate constant, k(red), which is equal to the rate constant measured at neutral pH. In contrast, for methionine similar plots show a marked departure from linearity which is exaggerated with increasing pH. At pH similar to 13.4 the slow process is entirely suppressed. These results in basic solutions of methionine are associated with the presence of a new transient tentatively assigned to an intramolecularly S therefore N-bonded radical cation of methionine and with the acid-base equilibria involving sulfur- and nitrogen-centered radical cations. Supplementary measurements of CO2 formation in steady-state photolysis experiments have revealed that with increasing pH there is a significant decrease in the value of the quantum yield of CO2 in the solutions of methionine and a minor decrease of the quantum yield of CO2 in the solutions of thiaproline. A detailed mechanism for the primary and the secondary photoreduction of CB is proposed and discussed.