The photochemical reactivity of poly(lactic acid) in the rubbery and molten state was determined by two methods: photoRheology and photoDSC. Significant changes in rheological behaviour of PLA were observed, depending on the viscoelastic regime. In the rubbery state, cold crystallization causes difficulties in oxygen diffusion resulting mainly in crosslinking reactions. In the molten state, the competition between crosslinking and degradation was observed due to hydroperoxides decomposition. In all cases UV irradiation intensifies the reactions. Using PhotoDSC we determined the effect of hydroperoxides on the PLA degradation process, and the difference in effectiveness of temperature and UV on hydroperoxides decomposition.