Abstract Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C 60 ) followed by 30 nm C 60 for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1,10-phenanthroline (Bphen) and 30 nm N , N ′-diphenyl- N , N ′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine p-doped with C 60 F 36 (MeO-TPD:C 60 F 36 ), which acted as hole transporting layer. Indium-tin-oxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H 2 18 O and 18 O 2 provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C 60 layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime.