A series of detailed studies is presented in which heat-induced interdiffusion is used to create a gradient bulk-heterojunction of 2-methoxy-5-(2(')-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) copolymer and C-60. Starting from a bilayer of spin-cast MEH-PPV and sublimed C-60, films are heated in the vicinity of the glass transition temperature of the polymer to induce an interdiffusion of polymer and fullerene. Variation of the polymer layer thickness shows that the photocurrents increase with decreasing layer thickness within the examined thickness regime as transport of the separated charges out of the film is improved. The interdiffusion was observed in situ by monitoring the photocurrents during the heating process and exhibited a rapid rise during the first five minutes. Cross-sectional transmission electron microscopy studies show that C-60 forms clusters of up to 30 nm in diameter in the polymer bulk of the interdiffused devices. This clustering of the fullerene molecules puts a significant constraint on the interdiffusion process that can be alleviated by use of donor-acceptor combinations with better miscibility.