We describe the adaptation of the recently developed thin-film rotating disk electrode method and its application in a rotating ring disk configuration (RRDE) to the investigation of the oxygen reduction reaction (orr) on a supported catalyst powder (PtVulcan XC 72 carbon). This allows the determination of kinetic data, such as reaction orders or apparent activation energies, for the orr directly without mathematical modeling. Collection experiments reveal a potential and rotation rate independent collection efficiency. RRDE measurements allow. for the first time, the direct determination of the fraction of peroxide production juring oxygen reduction on supported catalysts. Finally, comparison of measurements in 0.5 M H2SO4 and 0.5 M HClO4, respectively, reveals a significant effect of (bi)sulfate adsorption on the orr activity. On the basis of the present results, predictions are made on the kinetic limit of the orr in polymer electrolyte fuel tells, in the absence of ohmic and mass transport resistances at 100% utilization.