The current-voltage characteristics of methanofullerene [6,6]-phenyl C-61-butyric acid methyl ester (PCBM)-based devices are investigated as a function of temperature. The occurrence of space-charge limited current enables a direct determination of the electron mobility. At room temperature, an electron mobility Of mu(e) = 2 x 10(-7) m(2) V-1 s(-1) has been obtained. This electron mobility is more than three orders of magnitude larger than the hole mobility of donor-type conjugated polymer poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10-PPV). As a result, the dark current in PCBM/OC1C10-PPV based devices is completely dominated by electrons. The observed field and temperature-dependence of the electron mobility 1 of PCBM can be described with a Gaussian disorder model. This provides information about the energetic disorder and average transport-site separation in PCBM.