Triphenylphosphine (Ph(3)P) undergoes one-electron oxidation in irradiated CH2Cl2 solutions to yield the radical cation Ph(3)P(.+). This short-lived species exhibits intense absorption with a maximum at 325 nm and an extended shoulder at 400-500 nm and decays by a second-order process in the absence of O-2. In the presence of O-2, however, the radical cation reacts with O-2 with a rate constant of 7 x 10(6) L mol(-1) s(-1) to yield a peroxyl radical, Ph(3)P(+)OO(.), which exhibits no significant absorbance at lambda > 300 nm. Similar results, but with slightly different rate constants, were obtained also in CCl4 solutions and in CH3CN and cyclohexane solutions containing 1% CCl4. Tris(2,4,6-trimethoxyphenyl)phosphine in CH2Cl2 exhibits a behavior similar to that of Ph(3)P, but the reaction of its radical cation with O-2 is an order of magnitude more rapid. On the other hand, the perfluorinated Ph(3)P(.+) in CH2Cl2 reacts with O-2 much more slowly, if at all. Diphenyl-2-pyridylphosphine yields a radical cation which exhibits a slightly narrower absorption band but reacts with O-2 With the same rate constant as Ph(3)P(.+). The peroxyl radicals propagate a chain reaction by further oxidation of phosphine molecules to Ph(3)P(.+) either directly or indirectly. The final radiolysis product is the phosphine oxide, Ph(3)PO. The radiolytic yields for oxidation of the phosphines were much higher than the radiolytic yield of the solvent derived radicals, except for the case of the perfluoro derivative, and were dependent on the concentrations of phosphine and O-2 and on the dose rate. At low dose rates and high solute concentrations the chain lengths exceeded 1000.