Electrical conductivity measurements between 90° and 800°K have been performed on single crystals of uranium dioxide with compositions in the range UO2.000–UO2.007. The activation energy calculated from the ln&sgr;Tvs 1/Tcurves decreases from 0.34 to 0.19 eV with increasing excess oxygen content. Hall effect measurements by dc and ac techniques yielded no measurable response, and indicate that the Hall mobility is smaller than 0.015 cm2V−1sec−1at room temperature. This result and a consideration of the fundamental parameters determining the transport mechanism suggest an interpretation based on small polaron theory.