AbstractPhoto‐oxidative degradation of blends of poly(ethylene terephthalate) (PET) and poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) was studied considering the mutual influence of both components. The photo‐oxidation of these systems was investigated in the temperature range 30–100°C to study the effects of segmental motion of polymer chains. For short time of UV‐illumination at 365 nm the process is diffusion controlled and can be explained with the help of Waite's theory. The segmental interactions of polymer segments in the blends also affect the diffusion coefficients and the activation energy values. Higher values of the activation energy of PET in the blends cause a decrease of