Blends of poly-3-hexyl-thiophene (P3HT) and 6,6-phenyl-C61 butyric acid methyl ester (PCBM) are the most promising candidates for active layers of plastic solar cells with high efficiency in power conversion. The process which enables the nanostructuration of two interpenetrated continuous networks of the two materials in the blend is not fully understood. In this proposal we intend to measure the small angle neutron scattering (LOQ instrument) of solutions of PCBM, P3HT and a mixture of them at [1:1] weight ratio both in solution and thin films. The use of different solvents will be explored (toluene, dichlorobenzene and chloroform). Since PCBM is available in protonated and deuterated form, contrast matching will allow us to obtain useful information from the SANS measurements. Our aim is to understand the concentration fluctations and the aggregation processes in the P3HT:PCBM blends.