A non-isothermal, single phase membrane electrode assembly (MEA) mathematical model accounting for most applicable heat sources, viz., reversible, irreversible, ohmic heating, phase change, heat of sorption/desorption, is presented. The mathematical model fully couples a thermal transport equation with an MEA model and allows the study of non-isothermal effects, such as thermal osmosis through the membrane, local relative humidity variations in the catalyst layers and water sorption into the membrane. A detailed breakdown of various heat sources in the MEA at different current densities is provided and the impact of various thermal effects previously neglected in the literature such as thermal-osmosis, reversible heat distribution, and heat of sorption are studied. Results show that sorption heat cannot be neglected as it contributes up to 10% of the total heat under normal operating conditions. Reversible heat distribution can significantly affect the temperature distribution shifting the hottest location of the cell from anode and cathode. Analyzing the water transport across the membrane, results show that thermal-osmosis contributes up to 25% of the water flux inside the membrane at moderate and high current densities.