In this paper a study of the thermal performance of a commercial alkaline water electrolyzer (HySTAT from Hydrogenics) designed for a rated hydrogen production of 1 N m(3) H(2)/h at an overall power consumption of 4.90 kW h/N m(3) H(2) is presented. The thermal behaviour of the electrolyzer has been analyzed under different operating conditions with an IR camera and several thermocouples placed on the external surface of the main electrolyzer components. It has been found that the power dissipated as heat can be reduced by 50-67% replacing the commercial electric power supply unit provided together with the electrolyzer by an electronic converter capable of supplying the electrolyzer with a truly constant DC current. A lumped capacitance method has been adopted to mathematically describe the thermal performance of the electrolyzer, resulting in a thermal capacitance of 174 kJ degrees C(-1). The effect of the AC/DC converter characteristics on the power dissipated as heat has been considered. Heat losses to the ambient were governed by natural convection and have been modeled through an overall heat transfer coefficient that has been found to be 4.3 W m(-2) degrees C(-1). The model has been implemented using ANSYS (R) V10.0 software code, reasonably describing the performance of the electrolyzer. A significant portion of the energy dissipated as heat allows the electrolyzer operating at temperatures suitable to reduce the cell overvoltages. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.