We report on field effect mobility measurements in methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) films and in blends of poly(3-hexylthiophene) (P3HT) and PCBM, identical to those used in polymer solar cells. Electron mobilities in the order of 10-3cm2/Vs were found in the pure PCBM films, and electron mobilities of 10-4cm2/Vs were found in P3HT:PCBM blends at room temperature. Mobility measurements on the blends yielded electron mobilities of a factor 10 lower than those in the pure films. Temperature dependent measurements were performed to investigate the temperature dependence of the mobility in both films. In order to understand the discrepancy in the electron mobility between pure film and blend, we investigated the parasitic effects of the contacts on the measured value of mobility, but found that losses in the bulk dominate the current.