We present a steady-state two-dimensional model of a cross flow plate-and-frame membrane humidifier for a fuel cell system. Sensitivity analysis of the model is performed for various operating conditions and geometric parameters. The analysis shows that the water transfer rate increases with an increase in the velocities at the dry and wet sides, wet side inlet pressure, wet side inlet relative humidity, dry side inlet temperature and the number of plates. The relative humidity at the dry side outlet is found to increase with an increase in the wet side velocity, pressure at the dry and wet sides, wet side inlet relative humidity and the number of plates. Simulations are performed to study the performance of a system consisting of a humidifier and a fuel cell stack. It is observed that low current densities combined with a low stoichiometric ratio result in a high water transfer rate, water recovery ratio and relative humidity at the stack inlet. Maintaining a high operating pressure or using a high number of plates in the humidifier improves the performance of the humidifier, but they result in a respective increase in the parasitic losses, and the cost and size of the humidifier. (C) 2012 Elsevier B.V. All rights reserved.