The direct photochemical oxidation of liquid benzaldehyde in n-decane solution has been investigated at temperatures from 5 to 20 degrees C, the primary product being perbenzoic acid. The overall kinetics of the reaction are simple, the rate being proportional to the first power of the concentration of the aldehyde and to the square root of the intensity of the absorbed light. The rate is independent of oxygen pressure. These kinetics indicate that the dehydrogenation of the aldehyde by the peroxy radical is the rate-determining step in propagation. The termination step involves the interaction of two peroxy radicals. By means of the retardation and sector techniques the rate of initiation of the oxidation and the values of the rate constants have been determined.