The steady-state current-voltage performance and load cycle durability of polymer electrolyte fuel cells using Pt catalysts supported on Sn0.96Nb0.04O2-delta (Pt/Nb-SnO2) cathode catalyst layers (CLs), without any carbon additive, were evaluated with various ionomer contents. The apparent mass activity at 0.80 V (MA(app) @0.80 V) of the cell using Pt/Nb-SnO2 CL improved with decreasing volume ratio of the Nafion ionomer to the support (I/S). At I/S = 0.12, surprisingly, the MA(app) @0.80 V of the cell using Pt/Nb-SnO2 CL approached 2 times higher than that using a commercial Pt catalyst supported on GCB (Pt/GCB) CL with the optimized I/S ratio. The current density at 0.60 V of the cell using Pt/Nb-SnO2 (I/S = 0.12) CL also reached the same value of the cell using Pt/GCB CL. The electrochemically active surface area and MA(app) @0.80 V of Pt/Nb-SnO2 CLs during the load cycle durability test maintained higher values than those of Pt/GCB throughout the test, indicating that the Pt/Nb-SnO2 CL had higher durability than the Pt/GCB CL. Evaluation by means of low acceleration voltage transmission electron microscopy proved that the Nafion ionomer covered the hydrophilic surface of the Pt/Nb-SnO2 uniformly, whereas the ionomer did not do so on the hydrophobic surface of the Pt/GCB. The thin, uniform coverage of the Nafion ionomer, which was thus easily obtained on the Pt/Nb-SnO2 surface, was able to lower the overpotential originating from the oxygen diffusion resistance in the Nafion ionomer, thus improving the cell performance while maintaining the high load cycle durability. (C) The Author(s) 2018. Published by ECS.