An appropriate evaluation of oxide-based compound powders as non-platinum cathodes for polymer electrolyte fuel cells is required to investigate the active sites of oxide-based cathodes for the oxygen reduction reaction. A mixture of carbon black powder as an electro-conductive material with oxide catalysts that had low electrical conductivity was attempted to evaluate the oxygen reduction reaction (ORR) activity. An appropriate mixture of carbon black and the oxide catalysts let to the electrochemical activation of the oxide surface by the formation of electron, conduction networks. Using the carbon black mixture, the niobium-added titanium oxides heat-treated at 700 degrees C for 10 min under Ar containing 4% hydrogen without carbon materials were focused on to reveal the dependence of ORR activity on the added amount of niobium. The ORR activity of the niobium-added titanium oxides increased the Nb mole percentage from 0 to 20. This was due to the increase in the ratio of the Ti3+/(Ti4++Ti3+), and suggests that the active sites were Ti3+ and/or oxygen vacancies in the anatase phase. On the other hand, although the ratio of the Ti3+/(Ti4++Ti3+) of the oxide with an Nb mole percentage of 30 was almost twice as that with an Nb mole percentage of 20, the ORR activities were almost the same. This might be responsible for the deposition of the amorphous phase composed of Nb2O5, which consisted of a thin layer on the surface of the oxide particles and had low ORR activity. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.