The results of a study on the effects of encapsulant pre-conditioning (drying) on the durability of "conventional" and "inverted" bulk-heterojunction organic solar cells based on P3HT:PCBM are presented. The architectures of the conventional and inverted devices were ITO/PEDOT:PSS/P3HT: PCBM/Al and ITO/ZnO/P3HT:PCBM/MoO3/Ag, respectively. Quantitative analysis of the moisture content, moisture out-gassing, and re-absorbing properties of flexible barrier encapsulant films under ambient conditions was conducted. The effect of moisture out-gassing from the encapsulation materials on device performance was studied and the lifetime of conventional and inverted devices was found to decline significantly for the devices encapsulated using materials without pre-conditioning. This study reveals the essential role of pre-conditioning of materials in the encapsulation of organic photovoltaic devices. Current-voltage characteristics exhibit a clear increase in the series resistance of the devices during storage under ambient conditions, and Spectroscopic Impedance analysis indicates that charge-transport resistance within the devices increases significantly during the degradation process. The results are consistent with the primary degradation mechanism being corrosion at the metal-electrode surface. (C) 2014 Elsevier B.V. All rights reserved.