The effect of oxygen on the photodegradation of a polyurethane coating induced by accelerated UV irradiation was studied with two different light sources: 313-nm UVB and Xe lamps. Doppler-broadened energy spectra (DBES) and positron annihilation lifetimes were measured. as a function of incident positron energy (0-30 keV) and irradiation time (0-100 h). The photodegradation of the coating was characterized in terms of subnanometer defect changes. Significant variations of the S parameter (a defect parameter from DBES) and the ortho-positronium lifetime and intensity were observed as a function of oxygen concentration during exposure to UV irradiation. These results showed a significant enhancement in photodegradation due to the presence of oxygen, as indicated by a decrease in free volume and holes at the atomic level. Enhanced degradation in the presence of oxygen, as indicated by increased free-radical production (observed with electron spin resonance spectroscopy), was also observed.