In the current study, rotating disk electrode measurements were performed to gain insight into the oxygen reduction reaction (ORR) performance of Pt supported on single-phase tungsten monocarbide (WC) substrates. Using well-defined WC disks with precise Pt depositions, the interaction between Pt and WC can be studied during the ORR without any additional complexities, such as support effects and uncertainties regarding the direct contact of Pt with WC. Furthermore, XPS is used to determine the surface concentration and oxidation state of WC and Pt before and after ORR measurements, showing how the catalyst is affected during the ORR. The monolayer Pt on WC system shows smaller than expected activities compared to bulk Pt. The cause for the depressed activity is attributed to the formation of tungsten oxides under excess oxygen and high oxidizing potentials during the ORR. The presence of tungsten oxides are confirmed by XPS, and its effect on the stability of the monolayer Pt and WC system is further demonstrated with voltammetric results. These results clarify some unresolved issues with the current literature regarding the synergistic effect between Pt and WC in the ORR. (C) 2011 Elsevier B.V. All rights reserved.