Blends of fullerenes with conjugated polymers (CP), such as PCBM-P3HT blends, are being extensively investigated for polymer photovoltaic applications. We have been studying the mixing of P3HT-PCBM blends, as well as other CP-fullerene blends, in the solid state i.e. without the use of solvents. Our preliminary experiments using DSC and WAXS have shown that apparently a very high degree of mixing of PCBM and P3HT, can be induced in the solid state. In this proposal we wish to use SANS as an additional characterization technique. Our ultimate goal is to confirm if solid state processed and solution processed composites are identical as our preliminary DSC and WAXS results suggest. The results of this fundamental study are expected to lead to alternative methods of processing of CPs without the need for solvents making them more suitable for large scale production processes.