Experiments are conducted to evaluate the performance of a planar membrane humidifier for a PEM fuel cell stack. The humidification performance of the humidifier adopted here includes the dew point approach temperature (DPAT), water recovery ratio (WRR), and water vapor transfer rate (WVTR). Parametric studies involve the dry inlet temperature, the wet inlet dew point, and the flow rate across the humidifier. Results show that increasing the flow rate across the humidifier linearly increases its pressure drop. However, the DPAT increases sharply at high flow rates (>350 SLPM) due to the inadequate WVTR limited by the humidifier size. In addition, increasing the wet inlet dew point reduces the humidification performance by increasing the DPAT and reducing in the WRR. Moreover, an increase in the dry inlet temperature could reduce the DPAT at the low flow rate of 100 SLPM, but does not affect the DPAT at the high flow rate of 450 SLPM. Furthermore, the electrochemical performance of the stack humidifier assembly is only 10% less than that of the stack tested on a commercial test station. Stability test reveals a satisfactory reliability of the present membrane humidifier as well. (C) 2012 Elsevier B.V. All rights reserved.