This work reports the effect of niobium (0-1 at%) on photocatalytic activity and the band gap of TiO2 with controlled oxygen activity in the range 10(-12) Pa < p(O-2) < 10(5) Pa. It is shown that maximum of photocatalytic performance in partial water oxidation is observed for Nb-doped TiO2 containing 1 at% Nb that is processed at 1273 K in pure oxygen, p(O-2)=10(5) Pa. The mechanism of photocatalytic water oxidation is considered in terms of a predominant effect of photocatalytically active surface sites corresponding to titanium vacancies. It is shown that the niobium-induced increase of the band gap, surface potential and charge transport have a minor influence on photoreactivity of Nb-doped TiO2 with water. (C) 2016 Elsevier B.V. All rights reserved.