The lifetimes of organic photovoltaic cells based on conjugated polymer materials were studied. The device geometry was glass:ITO:PEDOT:PSS:C-12-PSV:C-60:aluminium. To characterise and elucidate the parts of the degradation mechanisms induced by molecular oxygen, 1802 isotopic labelling was employed in conjunction with time-of-flight secondary ion mass spectrometry. A comparison was made between devices being kept in the dark and devices that had been subjected to illumination under simulated sunlight (1000 W m(-2), AM1.5) and this demonstrated that oxygen-containing species were generated throughout the active layer with the largest concentration towards the aluminium electrode. For devices that had been kept in the dark oxygen species were only observed at the immediate interface between the aluminium and the organic layer. The isotopic labelling allowed us to demonstrate that the oxygen comes from the atmosphere and diffuses through the aluminium electrode and into the device. (c) 2005 Elsevier B.V. All rights reserved.