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Blends of a conjugated polymer and the soluble fullerene-derivative, PCBM, have been the most successful material combinations for making efficient solar cells by a solution-based process. Conversion efficiencies have recently climbed to nearly 10%. Key to these recent improvements in performance were, apart from chemical design of conjugated polymers, the improved control over the morphology of the bulk heterojunction. When thin films are prepared from solution, demixing occurs upon rapid solvent evaporation and determines the final nanostructure, which is influenced by the polymer-fullerene interactions, the molecules’ tendency to self-organise, and the kinetics of the film formation. Characterization of the composition and molecular orientation has been a challenging task, because of the requirements for chemical contrast and lateral and depth resolution. With near-edge absorption fine structure (NEXAFS) spectroscopy we have quantitatively determined the surface composition of polymer:fullerene blends. Differences in composition between surface and sub-surface confirm the vertical phase separation observed in such films by other techniques, e.g., SIMS, electron tomography, and neutron reflectometry. The dependence of the resonance peaks on the angle of incidence of the X-rays yields additional information about molecular orientation, and changes upon illumination reveals unprecedented information about the stability of these molecular materials.