The electrical conductivity and thermoelectric power of an uranium dioxide single crystal were measured between 908 and 1697 K and for P-O2, included between 10(-24) atm and the boundary UO2+x/U4O9. For T < 1273 K and near stoichiometry, the electrical conductivity versus P-O2, shows a plateau characteristic of an extrinsic regime. For T >= 1273 K, the conductivity exhibits a minimum. The Seebeck coefficient (Q) over bar (UO2), also exhibits a behavior change when the temperature is increased. For T < 1200 K, (Q) over bar (UO2), remains positive whatever the values of P-O2. For T > 1200 K, (Q) over bar (UO2), changes from negative to positive values when P-O2, increases. This set of results shows that the stoichiometric oxide presents a p extrinsic to intrinsic transition near 1273 K. At T <= 1273 K, the extrinsic conductivity values versus temperature confirm that the mobility of the holes occurs by a small polaron process, with an activation energy of 0.17 eV. At T > 1273 K, the electrical conductivity minima are characteristic of a p -> n transition. A band gap energy of 2.0 eV has been calculated from the temperature dependence of these conductivity minima values. Furthermore, this set of results has allowed us to determine the relative oxygen partial molar free energy of stoichiometric UO2