Abstract: Portable fuel cells are commonly operated in the dead-end mode because of such as high fuel utilization. However, the performance of such systems deteriorates continuously with an increase in the amount of by-products such as water vapor and nitrogen. In this study, to verify the effect of water vapor on Proton Exchange Membrane Fuel Cells (PEMFCs), constantload experiments were carried out for a current density of 600 mA/cm2 and a voltage of 0.4 V, respectively. The performance of the cell was more stable under constant voltage conditions than under constant current density conditions. Condensed water accumulated in the anode channel near the cell outlet. The experimental results show how the relative humidity (RH = 0.15, 0.4 and 0.75) of air at the cathode side affect the performance of PEMFCs with dead-end anode. At RH values higher than 0.15, the mean power density increased by up to 51% and the mean purge duration decreased by up to 25% compared to the corresponding initial values.