In this article, entropy production of ohmic heating and concentration polarization is investigated for two types of fuel cells (PEMFC and SOFC). Ohmic entropy production arises from resistance to electron flow through the electrodes, as well as ion flow through the electrolyte. Ohm's law is applied to both ion and electron flows, when formulating the entropy production. Also, entropy production arises from changes in concentration of the reactants, during fuel consumption at the electrode surfaces. Unlike past methods developed for a solid oxide fuel cell (SOFC), this article formulates entropy production within electrodes of a proton exchange membrane fuel cell (PEMFC). Predicted results of cell irreversibilities are successfully validated against measured data. The entropy based method provides a useful alternative to past schemes aiming to reduce voltage losses with polarization curves, as entropy production is directly governed by the Second Law and it encompasses both electrochemical and thermofluid irreversibilities within a fuel cell. (c) 2006 Elsevier Ltd. All rights reserved.