Exposure of MDMO-PPV and MDMO-PPV:PCBM to UV-visible light in deoxygenised atmosphere results in a slow evolution of the chemical structure of both components. A photodegradation mechanism of MDMO-PPV is proposed. Concerning MDMO-PPV:PCBM blends, the results indicated that the photodegradation of MDMO-PPV was significantly slowed down in the blend. Extrapolation of data obtained in the condition of artificially accelerated ageing suggests that, if well protected of oxygen (encapsulation), this active layer is photochemically stable for several years in real conditions of ageing. Moreover, the efficiently quenched fluorescence evidenced is promising concerning the charge separation all along the lifetime of the system. (c) 2008 Elsevier B.V. All rights reserved.