In this paper a novel and simple method to determine the O-2 reduction reaction activity of Pt/C in 99% H3PO4 at a temperature of 160 degrees C using a well-established thin film procedure is reported. It utilizes a disk of rotating rod made of glassy carbon as a working hydrodynamic electrode. By introducing polybenzimidazole as a thin film polymer binder, a modified rotating rod electrode disk (the cylindrical part is electrochemically inactive under the conditions used) suitable for measurements in hot concentrated H3PO4 was obtained. Using this electrode together with the Koutecky-Levich equation modified for rotating rod electrode hydrodynamics, the kinetics of the oxygen reduction reaction and mass transfer parameters of O-2 in 99% H3PO4 at 160 degrees C were successfully determined. The presented method can be easily used to determine the activity of various catalysts under conditions which preclude the use of a conventional rotating disk electrode. (C) 2017 Elsevier Ltd. All rights reserved.