We measured highly accurate ohmic resistances of single cells at high frequency (HF) using Pt/Nb-SnO2 cathodes under several different conditions, including various cathode potentials and various atmospheres, in order to study the relationships between the ohmic resistances of single cells and the electrical conductivity of the Pt/Nb-SnO2 catalyst for polymer electrolyte fuel cells (PEFCs). The ohmic resistance of a cell using Pt/Nb-SnO2 increased with increasing cathode potential, in the range of typical cathode operation (0.4 V to 1.0 V vs. the reversible hydrogen electrode). The increase of Pt loading on the Nb-SnO2 support from 9 wt% to 17 wt% was effective in decreasing the ohmic resistance and in improving the cell performance. We propose that Pt oxidation might impede the effect of shrinking the depletion layer of the Nb-SnO2 surface. The addition of graphitized carbon black (GCB) to the Pt/Nb-SnO2 cathode was able to improve the cell performance by constructing both electronic pathways and gas diffusion pathways in the catalyst layer, under low and high current density operation, respectively. The performance of the Pt/Nb-SnO2 + GCB cathode was superior to that of the Pt/GCB cathode due to these effects. (C) 2015 The Electrochemical Society.