The photoreactions of cysteine derivatives I with 4-carboxybenzophenone in D2O were investigated by measurements of chemically induced dynamic nuclear polarization (CIDNP). The quenching mechanism is electron transfer from sulfur at every pH; even if the amino group of I is deprotonated, electron transfer from nitrogen does not participate. Decarboxylation of I-.+ to give alpha-aminoalkyl radicals V-. occurs on the CIDNP time scale and causes strong cooperative effects. The decarboxylation rate is increased significantly by deprotonation of the amino function; this is due to product control. V-. decays by two competing pathways. Fragmentation of the C-beta-S bond in V-. yields vinylamine, which is hydrolyzed to acetaldehyde at pH less than or similar to 7.25, and thiyl radicals, which then attack the sensitizer to give combination products. Oxidation of V-. by ground-state sensitizer leads to sulfur-containing aldehydes or other products, depending on pH. Relative rates of fragmentation and oxidation were determined from CIDNP signal intensities. From the temperature dependence of the polarizations, the activation energy of beta-fragmentation was estimated to be 54 kJ mol(-1).