Photoinduced changes in solids films of P3HT and PCPDTBT are studied in order to elucidate the initial stage of photodegradation in polymer solar cells. We find that illumination of the polymers with either UV or visible light in the presence of air induces persistent radical cations on the polymer chains. The radical cations are identified by optical and electron paramagnetic resonance spectroscopy. The photoinduced charges are stable at room temperature for at least hours, but recombine rapidly upon removal of air from the sample chamber. We ascribe the persistent charges to P+-O2- charge pairs that are stabilized by chemical interactions between molecular oxygen and the polymer (P) Formation of persistent charges also takes place in the presence of the fullerene based electron acceptor PCBM.