The photooxidation of styrene-isoprene copolymers is investigated on the basis of infrared analysis of irradiated films. The results show that the polystyrene segments are stable under irradiation and degradation results only from the photooxidation of the polyisoprene segments. Oxidation of polyisoprene results in chain scissions and generates various carbonylated photoproducts. A three-dimensional structure is formed by cross-linking reactions resulting from additions on the double bonds and formation of ether linkage. In order to determine the influence of the polyisoprene content on the mechanism and the kinetics of oxidation, different compositions have been tested. The extent of degradation is found to be proportional to the polyisoprene content but the rate of photooxidation is constant whatever the composition. The distributions of the oxidation products within the polymer are similar whatever the composition. This behavior can be attributed to the incompatibility between the polyisoprene and the polystyrene segments, which leads to a phase demixion and the formation of nodules. Each nodule can be considered as an independent photochemical reactor.