The device function of polymer bulk heterojunction (BHJ) solar cells has been commonly interpreted to arise from charge separation at discrete interfaces between phase-separated materials and subsequent charge transport through these phases without consideration of phase purity. To probe composition, the miscibility of poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) with phenyl-C61-butyric acid methyl ester (PCBM) has been determined, while the effects of polymer crystallinity on miscibility are probed using P3HT grades of varying regioregularity. It is found that, while no intercalation occurs in P3HT crystals, amorphous portions of P3HT and MDMO-PPV contain significant concentrations of PCBM, calling into question models based on pure phases and discrete interfaces. Furthermore, depth profiles of P3HT/PCBM bilayers reveal that even short annealing causes significant interdiffusion of both materials, showing that under no conditions do pure amorphous phases exist in BHJ or annealed bilayer devices. These results suggest that current models of charge separation and transport must be refined.Keywords (keywords): moleculear miscibility; polymer-fullerene blends; organic solar cells; thermodynamic phase diagram; near-edge X-ray absorption fine structure (NEXAFS); grazing-incidence wide angle X-ray scattering (GIWAXS or GIXRD); dynamic secondary ion mass spectrometry (SIMS)