The effect of photoactive pigments such as ZnO, CdS, and TiO2 on the photoorddation ( with and without water vapor) of ethylene - propylene rubber and atactic polypropylene were examined. The effects of surface treatment (passivation) and low granulometry (nanopigments) were also studied for TiO2. A drastic increase of ketone and an inhibition of carboxylic acid formation for ZnO filled polymers were revealed and quantitatively measured by using derivatization reactions. Such a phenomenon, as well as a small decrease of average molecular weight, were explained by a modification of the ZnO decomposition mechanism of tertiary hydroperoxides.