The time-of-flight method has been used to study the effect of P3HT molecular weight (M-n = 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) cm(2) V-1 s(-1) its 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 front these blend films.