The time-of-flight method has been used to study the effect of P3HT molecular weight (Mn = 13–121 kDa) on charge mobility in pristine and PCBM blend films using highly regioregular P3HT. Hole mobility was observed to remain constant at 10−4 cm2V−1s−1 as molecular weight was increased from 13–18 kDa, but then decreased by one order of magnitude as molecular weight was further increased from 34–121 kDa. The decrease in charge mobility observed in blend films is accompanied by a change in surface morphology, and leads to a decrease in the performance of photovoltaic devices made from these blend films.