The adsorption of a porphyrazine (a tetraazaporphyrin) on the surfaces of Pt nanoparticles enhanced the activity of the oxygen reduction reaction (ORR) of the Pt catalysts for use in a polymer electrolyte fuel cell (PEFC). The apparent electrochemically active surface area was significantly decreased upon adsorption of the organic macrocycle. The promotion of the ORR was ascribed to the enhanced activity of the residual Pt surface area. Cyclic voltammograms under argon atmosphere and CO stripping voltammetry indicate that the porphyrazine impedes the generation of OH adsorbed on the surface of Pt. Other pi-conjugated aromatic compounds tested did not increase (but rather, inhibited) the ORR activity. It should be noted that the porphyrazine, comparably large in size relative to the Pt nanoparticles, enhanced the catalytic activity of the Pt surface. (C) 2018 Elsevier Ltd. All rights reserved.