This article is devoted to the study of the photoaging and thermal aging of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV; also called OC1C10-PPV) used in organic solar cells. Thin MDMO-PPV films (thickness < 1 μm) were exposed to ultraviolet-light irradiation (λ > 300 nm) in the presence of air or thermooxidized at 60°C. The modifications of the chemical structure of the matrix were analyzed with ultraviolet-visible and infrared spectroscopy. The oxidation products that formed were identified by postirradiation treatments, including chemical derivatization reactions. On the basis of the identification of the various products formed, a two-step radical mechanism is proposed to account for the modification of the chemical structure of the polymeric matrix. It involves first the oxidation of the ether substituent followed by the oxidation of the double bonds. These reactions are responsible for a loss of conjugation of MDMO-PPV, chain scissions, and a decrease in the visible absorbance, which are anticipated to drastically impair the photovoltaic properties of the material.