A clear dependence of the hole mobility on molecular weight of poly(3-hexylthiophene) in films is demonstrated. The charge carrier mobility of regioregular polythiophene is shown to increase by four orders of magnitude as the molecular weight is increased from similar to3 200 g mol(-1) to 36 000 g mol-1. These observations could explain why the mobility values obtained in different labs vary so widely and suggest that optimizing the molecular weight of conjugated polymers could lead to significant improvements in device performance.