In this paper, we have shown that an increase in the pressure of oxygen on the cathode side in HT-PEM fuel cells leads first to an increase and then a decrease in the potential of the cell and again at even higher pressures to an increase. The most likely explanation for this peculiar phenomenon is a competition between oxygen and phosphoric acid on the surface of the platinum catalyst in the three phase boundary. An increase in the oxygen pressure will in this way remove phosphoric acid from the platinum giving more space for oxygen but at higher pressure creating mass transport problems due to lack of phosphoric acid whereas at even higher pressures oxygen can orient itself in a different way on the platinum surface giving more space to the acid. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.