The reaction of chloromethyl radical with molecular oxygen in solid argon has been studied using matrix isolation infrared absorption spectroscopy. The chloromethyl radical was produced via codeposition of chloromethane with high frequency discharged argon atoms. The chloromethyl radical reacted with dioxygen spontaneously on annealing to form the chloromethylperoxy radical, which was characterized on the basis of isotopic splitting (CO and O) as well as quantum chemical calculations. The chloromethylperoxy radical dissociated to give the weakly bound CICO-H2O complex under UV light irradiation. The calculations predicted that the CICO-H2O complex was produced by hydrogen atom transfer from a CHCI(O)-OH intermediate.