The oxygen reduction process in polymer electrolyte fuel cells (PEMFCs) was in-situ investigated at intermediate temperatures (80 degrees-130 degrees C) by using a carbon supported PtCo catalyst and Nafion membrane as electrolyte. To overcome the Nafion dehydration above 100 degrees C, the experiments were carried out under pressurized conditions. Electro-kinetic parameters such as reaction order and activation energy were determined from the steady-state galvanostatic polarization curves obtained for the PEM single cell. Negative activation energies of 40 kJ mol(-1) and 18 kJ mol(-1) were observed at 0.9 V and 0.65 V, respectively, in the temperature range 100 degrees-130 degrees C. This was a consequence of ionomer and membrane dry-out. The ionomer dry-out effect appears to depress reaction kinetics as the temperature increases above 100 degrees C since the availability of protons at the catalyst -electrolyte interface is linked to the presence of proper water contents. An oxygen reduction reaction of the first order with respect to the oxygen partial pressure was determined at low current densities. Maximum power densities of 990 mW cm(-2) and 780 mW cm(-2) at 100 degrees C and 110 degrees C (H-2-O-2) with 100% R.H., were achieved at 3 bars abs. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.