Abstract The contribution of humidity to the photodegradation of a commercial bisphenol A polycarbonate (BPA PC), aged under QUV-A radiation for periods of up to 3000 h, at relative humidity values of 0 and 42%, was investigated. The mechanism of degradation was identified as the competition of photo-Fries and photo-oxidation, with photo-oxidation predominating as exposure time increased. The molecular weight of polycarbonate was seen to change during UV exposure, making it susceptible to embrittlement and surface erosion. This coupled with the discolouration of the polymer contributed to the loss of optical properties, including reduced light transmission, loss of gloss, increased haze and discolouration. Photo-induced yellowing was a self-accelerating process. It was initially accelerated by humidity; but after longer exposures it was humidity controlled, with the process continuing at lower humidity whilst at higher humidity photo bleaching occurred. The reduction in other optical properties was more severe in a low humidity environment. Samples exposed to humidity without UV showed no changes.