To obtain insights into the roles of carbon and nitrogen contained in the starting materials in partially oxidized tantalum carbonitride (TaCxNy) oxygen-reduction electrocatalysts, we studied oxygen-reduction-reaction (ORR) behaviors and structural properties of tantalum-oxide-based catalysts prepared by partial oxidation of tantalum carbides (TaC) and nitrides (TaN) under low oxygen pressure with 2%H-2/N-2 based gas at 1000 degrees C. The surface tantalum-oxide phase obtained from TaC were beta-Ta2O5 in whole degree of oxidation (DOO) range, while the both TaON and Ta2O5 were formed in the surface phases obtained by the oxidation of TaN, especially at low DOO regions. The highest ORR response was obtained for beta-Ta2O5 containing partially oxidized TaC (Ta-COs). Although, the local structure of surface oxide phase of beta-Ta2O5 containing partially oxidized TaN (Ta-NOs) are almost identical, Ta-NOs showed no significant ORR responses. Carbon contained in the starting materials, therefore, plays an indispensable role in emergence of ORR activity. (C) 2012 Elsevier Ltd. All rights reserved.