We report on studies of device degradation in organic photovoltaic devices based on blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Since delamination, oxidation, and chemical interactions at the metal electrode/ organic interface have long been posited as degradation pathways in organic electronic devices, we first investigated the stability of a variety of electrodes for devices stored in an inert, dark environment. Second, a set of experiments was designed to separate the effects at the metal/organic interface from the degradation of the active layer or the hole extraction interface. To do this, Ca/Al electrodes were deposited to complete half of a substrate's devices, and samples were left both under constant illumination and 10% illumination (10% duty cycle of I sun illumination) in a glovebox environment. After more than 200 h of measurement, additional electrodes were deposited and device performance of each set was compared. Third, to assess the degree of degradation caused by photo-induced processes, device stability in an inert atmosphere under constant illumination, and 10% illumination conditions was also investigated. Last, various degradation mitigation strategies in air under constant illumination were explored. The results showed that the active layer itself is not inherently unstable on the timescales studied here. Choosing the appropriate electrode (Ca/Al) reduced interfacial degradation, storing the active layer in an inert, dark environment did not cause significant degradation, and storing the active layer under constant illumination caused only a limited reduction in performance. Our results indicate that the metal/organic interface can be a significant source of degradation in the devices, and we discuss approaches that could reduce this instability. (c) 2008 Elsevier B.V. All rights reserved.