The photooxidation and photolysis of poly(butylene terephthalate)-fibre glass (PBT-FG) were studied, and a comparison with the photochemical behaviour of PET was made. It was found that the same photoproducts were formed in PET and PBT-FG, and that fibre glass had no influence on the different routes involved in the mechanisms of photolysis and photooxidation. The determination of the photoproduct concentration profiles in PBT-FG and PET showed that photooxidation and photolysis were limited to a depth of 50 mu m in the irradiated surface. Moreover, it was found that the ratio between the rates of formation of the photoproducts in PBT-FG and PET was equal to the ratio between the contents of PET in PBT-FG and pure PET.