We synthesized Pt catalysts supported on titanium nitride nanoparticles (Pt/TiN. Pt loading, 8.9 wt%) by a colloidal method. The Pt nanoparticles were found to exhibit a characteristic hexahedral shape with clear faceting, with a lattice structure that was highly oriented to that of the TiN support, indicating a strong interaction between the Pt nanoparticles and the TiN support. We evaluated the electrochemical activity of the Pt/TiN both without and with mixing with electronically conducting acetylene black (AB), the latter denoted as Pt/TiN + AB. The electrochemically active surface area (ECSA) of Pt in Pt/TiN + AB was noticeably larger than that of Pt/TiN, reaching 108 m(2)g((pt))(-1) as a maximum, which was close to the geometrically estimated Pt surface area (127 m(2) g((pt))(-1)), based on transmission electron microscopy. By means of a potential step cycling test (0.9-1.3 V vs. RHE) simulating start/stop cycles in polymer electrolyte fuel cells (PEFCs), it was found that both of the TiN-based catalysts exhibited superior durability compared to commercial Pt catalysts supported on carbon black (Pt/CB) or graphitized carbon (Pt/GCB), based on the ECSA values. The mass activities at 0.85 V of both of the TiN-based catalysts for the oxygen reduction reaction (ORR) were also higher than those for commercial Pt/CB or Pt/GCB during the potential step cycling test. The superiority of the TiN catalysts must have arisen as a result of the stabilization of the Pt nanoparticles due to their strong interaction and well matched crystallographic structures, as well as the complete elimination of nanopores from the support surfaces. (C) 2012 Elsevier Ltd. All rights reserved.