The phase diagrams for three conjugated polymer/fullerene blends of interest for polymer solar cells, namely semicrystalline poly(3-hexylthiophene) (P3HT):methanofullerene [6, 6]-phenyl C61-butyric acid methyl ester (PCBM), poly(2-methoxy-5-(3', 7'-dimethyloctyloxy)-p-phenylenevinylene) (MDMO-PPV):PCBM, and poly-(2-methoxy-5-(2'-ethylhexyloxy)-p-phenylenevinylene) (MDMO-PPV):PCBM, have been constructed based on X-ray scattering data and differential scanning calorimetery (DSC). Both melting point depression and glass transition temperature elevation were observed in the P3HT:PCBM blends as a function of increasing PCBM wt %. The PCBM solubility limit, i.e., the phase-separation point, was determined to be 30, 40, and 50 wt % PCBM for P3HT:PCBM, MDMO-PPV:PCBM, and MEH-PPV:PCBM mixtures, respectively. The phase behavior of the blends is directly correlated with electrical transport behavior determined by measuring field effect conduction in a transistor testbed. Specifically, below the solubility limit for PCBM in all three blends, only hole transport was observed, and above the solubility limit both hole and electron conduction were measured.