A study of the photo-oxidation of films of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) blended with [6,6]-phenyl C-61-butyric acid methyl ester (PCBM), and solar cells based thereon, is presented. Solar-cell performance is degraded primarily through loss in short-circuit current density, J(SC). The effect of the same photodegradation treatment on the optical-absorption, charge-recombination, and charge-transport properties of the active layer is studied. It is concluded that the loss in J(SC) is primarily due to a reduction in charge-carrier mobility, owing to the creation of more deep traps in the polymer during photo-oxidation. Recombination is slowed down by the degradation and cannot therefore explain the loss in photocurrent. Optical absorption is reduced by photo-bleaching, but the size of this effect alone is insufficient to explain the loss in device photocurrent.