The degradation of organic photovoltaic cells (OPVs) based on pentacene/fullerene heterojunction was investigated in air with different relative humidity (RH). The lifetime of OPVs stored in air with 33 +/- 5% RH was about 850 h, 35 times of those stored in air with 65 +/- 5% RH (25 h). Two degradation processes were clearly observed for the devices in low humidity. The results demonstrate that the water and oxygen have different effects on pentacene/fullerene heterojunction OPVs degradation, thus dominating at different time periods. The different decay constants caused by O(2) and H(2)O might come from the diffusion of O(2) and H(2)O, respectively. Water is believed to be more important in the degradation process of pentacene/C(60) based solar cells. (C) 2010 Elsevier B.V. All rights reserved.