This study was conducted to numerically investigate the effects of phosphoric acid (PA) loss and electrochemical surface area (ECSA) reduction on the voltage degradation of high temperature proton exchange membrane fuel cells (HT-PEMFCs) during long-term operations. A three-dimensional HT-PEMFC model developed in our previous studies was used for the simulations, in which the PA doping level was continuously reduced over time to capture the voltage degradation rate measured during constant current-based long-term HT-PEMFC operations. The rate of PA loss was then estimated and compared with the measured values of PA loss in the literature. The rate of PA loss predicted by the model was at least one order of magnitude higher than the experimental values of PA loss, which implies that the voltage degradation was mainly caused by the ECSA reduction even under the constant current operations. Indeed, better agreement with the experimental data was achieved when the effects of both the ECSA reduction and PA loss were taken into account during the simulations. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.