A calorimeter has been constructed and used to measure the total heat production of a single polymer electrolyte fuel cell that is operated on hydrogen and oxygen at 50°C and 1bar. The cell had a SolviCore Catalyst Coated Backing and Nafion membranes 112, 115 and 117. We report that the total heat production plus the power production corresponds to the enthalpy of formation of water for cell potentials above 0.55V. For cell potentials less than 0.55V, we measured a linear decrease in the reaction enthalpy with decreasing cell potential. This effect was obtained independently of membrane thickness and current density. We propose therefore that the main power loss at low cell potentials and the inflection point in the polarisation curve is due to hydrogen peroxide formation at the cathode. The total heat production was decomposed into reversible and irreversible effects (non-ohmic and ohmic). The non-ohmic part was evaluated using Tafel plots. We show that it is possible to determine the overpotential of an electrode also from its thermal signature.