|Developing a better understanding of the evolution of morphology in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies greater than 10%. In the most extensively characterized system, the poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl-C-61-butyricacid-methyl-ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphology during processes such as thermal annealing are not well understood. In this work, we use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. We find that PCBM aggregates and/or molecular species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the favorability of mixing in this system, opposing the belief that phase-pure domains form in BHJs due to immiscibility of these two components.