This paper presents the thermodynamic modelling of a proton exchange membrane (PEM) fuel cell at various operating conditions through energy and exergy analyses. In addition, the correlations and equations available in literature were used to determine thermodynamic irreversibilities in the PEM fuel cell at different operating conditions such as cell temperature, pressures of anode and cathode, current density, and membrane thickness as a design parameter. In accordance with the practical conditions, it is considered that the anode and cathode pressures range from 3 to 5 atm; cell temperature from 50 to 80°C. Also, the membrane thickness is chosen as 0.016, 0.018 and 0.02cm, respectively. Depending on these data, thermodynamic irreversibilities throughout PEM fuel cell operation are estimated and discussed in detail. Consequently, it is found that thermodynamic irreversibilities in fuel cell increased with a rise of membrane thickness and with a decrease of the cell temperature and pressure. © 2006 Inderscience Enterprises Ltd.