The photooxidation of four different polyethylenes with various amounts of chemical defects was examined. These polyethylenes had different initial contents of vinyl and vinylidene groups, and two of them exhibited an infrared absorption band attributed to ketones. The rates of photooxidation of these four polymers were rather different, and the rates of photooxidation increased with the initial concentration of ketones. The ketones were tentatively attributed to the use of propanal as a transfer agent during polymer synthesis and were readily eliminated by photolysis (irradiation in vacuum) of the samples. The identification of butanone formed by Norrish-II reactions confirmed the presence of propanal. Notably, the four samples behaved similarly after prephotolysis and degraded at the same rate. Moreover, the rates of photooxidation after prephotolysis were found to be totally independent of the concentrations of unsaturations, which does not confirm the hypothesis of a potential influence of such groups on the initial photostability of polyethylene.