Polymer/fullerene blends represent the most common form of organic solar cell. This proposal forms part of a wider effort in which our motivation is to complement solar cell optimisation strategies, with in-depth studies aimed at increasing the fundamental understanding of the materials science within polymer/fullerene thin films. Long-term stability is a critical issue for the commercialisation of organic devices, and initial measurements suggest considerable sensitivity of the physical structure/morphology of films to aging processes. Our aim in this proposal is to understand how various aging processes can affect miscibility in polymer/fullerene thin-films, and to determine the origin of this sensitivity. We will investigate this by performing chemical and physical aging experiments, of direct relevance to device operating conditions, followed by specular reflectivity measurements.