High electron mobility and ambipolar charge transport are observed in phase-separated binary blends of n-type poly(benzobisi-midazobenzophenanthroline) (BBL) with p-type polymer semiconductors, poly[(thiophene-2,5-diyl)-alt-(2,3-diheptylquinoxaline-5,8-diyl)] (PTHQx) and poly(10-hexylphenoxazine-3,7-diyl-alt-3-hexyl-2,5-thiophene) (POT). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) show phase-separated domains of 50-300 nm in the binary blend thin films. The TEM images and electron diffraction of BBL/PTHQx blends show the growth of single-crystalline phases of PTHQx within the BBL matrix. A relatively high electron mobility (1.0 X 10(-3) cm(2) V-1 S-1) that is constant over a wide blend-composition range is observed in the PTHQx blend field-effect transistors (FETs). Ambipolar charge transport is observed in both blend systems at a very high concentration of the p-type semiconductor (>= 90 wt % PTHQx or >= 80 wt % POT). Ambipolar charge transport is exemplified by an electron mobility of 1.4 x 10(-5) cm(2) V-1 s(-1) and a hole mobility of 1.0 x 1.0 (-4) cm(2) V-1 S-1 observed in the 98 wt % PTHQx blend FETs. These results show that ambipolar charge transport and the associated carrier mobilities in blends of conjugated polymer semiconductors have a complex dependence on the blend composition and the phase-separated morphology.