We investigate the performance and stability of a platinum (Pt) supported on indium tin oxide (ITO) electrocatalyst. ITO was synthesized using the co-precipitation method and uniform particles with a B.E.T. surface area larger than 40m2/g were obtained. Pt was dispersed onto the ITO by colloidal deposition followed by reduction with formaldehyde. Previous rotating disk electrode (RDE) work has shown that Pt/ITO possesses high activity and stability for the oxygen reduction reaction (ORR). However, this catalyst exhibited very poor performance and stability in an operating polymer electrolyte fuel cell (PEFC) membrane electrode assembly (MEA). For H2/O2 PEFC operation at 80°C and 75% relative humidity (RH), the current density obtained at 0.55V was only 90mA/cm2, and the maximum current density was only 150–160mA/cm2, when Pt/ITO was used at the cathode, whereas a limiting current density of 3900mA/cm2 was readily obtained using a benchmark Pt/C catalyst under identical conditions. The low performance with Pt/ITO was primarily due to the high overall cell resistance of the MEA (>400mOhmcm2). X-ray photoelectron spectroscopy (XPS) was employed to investigate the degradation of the Pt/ITO catalyst in the PEFC electrode during operation. The deconvolution of the indium 3d XPS peak revealed the presence of two peaks: the first was assigned to indium oxide in ITO (at 445.6eV), while the second was assigned to indium hydroxide (at 446.6eV). We observed an increase in surface hydroxide concentration (compared to pristine Pt/ITO) after the Pt/ITO catalyst was used either at the cathode or anode of an operating PEFC. We postulate that the surface hydroxides form a passivating layer that increases the electrode resistance and undermines PEFC performance. •Contrary to RDE results, low performance was observed with a Pt/ITO electrocatalyst in an operating PEFC.•X-ray photoelectron spectroscopy (XPS) revealed Pt/ITO catalyst degradation during PEFC operation.•Indium 3d XPS peak deconvolution revealed the degradation mechanism, namely the formation of surface indium hydroxides.•We postulate that the surface hydroxides form a passivating layer that increases the electrode resistance and undermines PEFC performance.