The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripeptides (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate constants for quenching of the CB triplet by sulfur-containing peptides were determined to be in the range (1.8-2.3) x 10(9) M(-1) s-(1) for neutral and alkaline solutions. The presence of the various electron-transfer intermediates accompanying the CB triplet quenching events was identified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. The intermediates identified were the CB ketyl radical anion, the CB ketyl radical, intermolecularly (S therefore S)-bonded radical cations, and intramolecularly (S therefore N)-bonded radical cations derived from peptides. The spectra of appropriate (S therefore S)(+) and (S therefore N)(+) intermediates for the peptides were determined from complementary pulse radiolysis studies in acidified aqueous solutions of the peptides. The types of intermediates were found to depend on the pH of the solution and on the location of the methionine unit with respect to the terminal functions. The quantum yields of all the transients and the kinetics of their formation and decay were measured by flash photolysis, and quantum yields of CO2 formation were measured by steady-state photolysis. These results were based on the resolution of the spectral components in the transient absorption spectra at various delays after the flash. A detailed mechanism of the CB-sensitized photooxidation of methionine-containing peptides was discussed and compared to that for methionine.