Abstract The effect of accelerated UV‐aging on properties of PP/wood‐fiber composites has been studied. Plates containing 0, 25 and 50 wt% wood‐fibers were subjected to accelerated UV‐aging in a QUV weatherometer for up to 8 weeks. Stabilization against UV‐degradation was made by adding 3% of a UV stabilizer. The results showed that both the unfilled PP and the PP/wood‐fiber composites displayed good UV resistance with regard to mechanical properties. The color of the PP/wood‐fiber composite plates changed from brown to chalky white at the exposed area because of the formation of a thin, strongly degraded surface layer. Physical and chemical analysis of these surface layers using differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy showed that degradation of the PP matrix had occurred, leading to chemicrystallization and extensive surface cracking. A decrease in PP melting temperature was also noted, due to molecular chain scission and the formation of extraneous groups, such as carbonyls and hydroperoxides. These results indicate that the wood‐fibers act as chromophores when incorporated in the PP matrix. Increasing the fiber content from 25 to 50 wt% increases the rate of degradation of the PP matrix by approximately a factor of two. Polym. Compos. 25:543–553, 2004. © 2004 Society of Plastics Engineers.