The data of electrochemical experiments, analysis of solutions, transmission electron microscopy and scanning tunneling microscopy are applied to separate various processes resulting in degradation of carbon supported platinum cathode material in hot concentrated phosphoric acid in the presence of polybenzimidazole. Under these modelling HT-PEFC operation conditions, dissolution of metallic platinum is found to play a minor role as compared to sintering of nm-size platinum crystals. Yet, polybenzimidazole significantly affects platinum dissolution, which results in up to 2 orders of magnitude increase of integral rate of potentiostatic dissolution at 0.87 V vs RHE. (C) 2015 The Electrochemical Society. All rights reserved.