in order to model accurately heat transfer in PEM fuel cell, a particular attention had to be paid to the assessment of heat sources in the cell. Although the total amount of heat released is easily computed from its voltage, local heat sources quantification and localization are not simple. This paper is thus a discussion about heat sources/sinks distribution in a single cell, for which many bold assumptions are encountered in the literature. The heat sources or sinks under consideration are: (1) half-reactions entropy, (2) electrochemical activation, (3) water sorption/desorption at the GDL/membrane interfaces, (4) Joule effect in the membrane and (5) water phase change in the GDL A detailed thermodynamic study leads to the conclusion that the anodic half-reaction is exothermic (Delta S-rev(a) = -226 J mol(-1) K-1), instead of being athermic as supposed in most of the thermal studies. As a consequence, the cathodic half-reaction is endothermic (Delta S-rev(c) = +62.8 J mol(-1) K-1), which results in a heat sink at the cathode side, proportional to the current. In the same way, depending on the water flux through the membrane, sorption can create a large heat sink at one electrode and an equivalent heat source at the other. Water phase change in the GDL - condensation/evaporation - results in heat sources/sinks that should also be taken into account. All these issues are addressed in order to properly set the basis of heat transfer modeling in the cell. (C) 2009 Elsevier B.V. All rights reserved.