This paper describes the construction of a phase diagram for the as-cast state in the organic photovoltaic system P3HT:PCBM. Evidence for a transition to a phase-separated state at PCBM concentrations greater than 70 wt % is seen both by DMTA and GIWAXS, and the glass transition temperatures of blends in the single phase state below 70 wt % PCBM are observed to be raised compared to the pure polymer. Pure PCBM is observed to exhibit a thermal transition at 155 °C, an observation unreported to dateoffering insight into crystallites commonly seen in device films. The liquid-crystal phase of P3HT is shown to persist in the presence of up to 41 wt % PCBM. In addition, pure PCBM is shown to be significantly hygroscopic, with important implications for the processing of high-performance devices.