The effects of the operating backpressure on PEM fuel cell reactions and performance are analyzed using both theoretical and semi-empirical approaches. The change in fuel cell operation backpressure can affect the reversible thermodynamic voltage, exchange current density of the electrode reactions, the membrane conductivity, as well as mass transfer limiting current densities. Backpressure can affect the fuel cell open circuit voltage (OCV) by affecting theoretical potential, mixed O-2-Pt voltage drop as well as the H-2 crossover induced voltage drop. The magnitude of the fuel cell voltage change with backpressure in both electrode kinetic and mass transfer ranges is determined mainly by both the fuel cell reaction exchange current densities and the mass transfer limiting current densities. Theoretical modeling indicates that when the fuel cell is controlled at a low backpressure or backpressure range, the change in backpressure will lead to a larger fuel cell voltage change than that of a fuel cell controlled at a high backpressure range. Overall, an increase in the fuel cell backpressure will result in a performance enhancement such as higher open circuit voltage, and kinetic cell voltages at higher backpressures. However, negative effects such as high crossover, sealing problems, parasitic power losses, higher cost for compression and enlarged volume of the fuel cell system can occur with a higher operating backpressure. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.