Two series of binary blends of regioregular poly(3-hexylthiophene) (PHT) with a polymer semiconductor, poly [2-methoxy-5-(2-ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV), and with an insulating polymer, polystyrene, were investigated and found to be phase separated and to exhibit relatively high field-effect mobility of holes ((0.1-6) x 10(-3) cm(2)/(V s)). Atomic force microscopy showed nucleation and growth type phase separation on the length scale of 100-600 nm in the binary blend systems. The mobility of holes in PHT/polystyrene blends is up to 7-fold higher than in PHT/MEH-PPV blends. The reduced carrier mobility in the PHT/MEH-PPV blends is due to dipolar effects arising from the polarity of the components. These results show for the first time that dipolar effects can substantially modify the charge transport properties of blends of conjugated polymers.