The performance of bulk heterojunction solar cells (BHSC) strongly depends on the molecular morphology of the similar to100 nm thick spin cast photoactive films of the polymer/fullerene composites used. Using dynamic TOF-SIMS, we show here that spontaneous stratification is absent for several methanofullerene concentrations by use of a deuterium labeled methanofullerene in composite films of a poly(p-phenylene vinylene) and a methanofullerene. In addition, imaging TOF-SIMS on annealed samples shows a dramatic increase in phase segregation on a sub-micron level. The TOF-SIMS results are supported by transmission electron microscopy (TEM) and atomic force microscopy (AFM) measurements. (C) 2004 Elsevier B.V. All rights reserved.