Tantalum oxynitrides (TaOxNy) and nitride (Ta3N5) have been studied for application as a cathode catalyst of polymer electrolyte fuel cells (PEFCs) without platinum. The nitride and oxynitrides in which Ta had the highest oxidation state showed the highest stabilities in sulfuric acid. Ta3N5 showed poor activity for the oxygen reduction reaction. TaO0.92N1.05 (nitrogen content: 7.0 wt %) had a clear catalytic activity for the oxygen reduction reaction in sulfuric acid (1 mol/dm(3)) at 30 degrees C, although its activity is presently much lower than that of platinum. (C) 2005 The Electrochemical Society.