The present work reports the electrical conductivity and thermoelectric power for Nb-doped TiO2 at elevated temperatures (1073-1298 K) in the gas phase of controlled oxygen activity, 10(-14) Pa < p(O-2) < 75 kPa. It is shown that in reduced conditions the Nb-doped TiO2 exhibits metallic-type conductivity. This finding paves the way for the development of high-performance photoelectrodes with substantially reduced internal energy losses during charge transport. The present work also determined the equilibrium constant for the formation of oxygen vacancies and titanium vacancies for Nb-doped TiO2.