This article is dedicated to study the interlinked effects of symmetric relative humidity (RH), and asymmetric RH on the performance of H-2/air PEM fuel cell at different temperatures. The symmetric and asymmetric RH were achieved by setting the cathode relative humidity (RHC) and anode relative humidity (RHA) as equal and unequal values, respectively. The cell performance was evaluated by collecting polarization curves of the cell at different RH, RHC and RHA and at different cell temperatures (T-cell). The polarization curves along with the measured internal cell resistance (membrane resistance) were discussed in the light of the present fuel cell theory. The results showed that symmetric relative humidity has different impacts depending on the cell temperature. While at RH of 35% the cell can show considerable performance at T-cell = 70 degrees C, it is not so at T-cell = 90 degrees C. At T-cell = 70 degrees C, the cell potential increases with RH at lower and medium current densities but decreases with RH at higher currents. This was attributed to the different controlling processes at higher and lower current densities. This trend at 70 "C is completely destroyed at 90 degrees C. Operating our PEM fuel cell at dry H, gas conditions (RHA = 0%) is not detrimental as operating the cell at dry Air (02) conditions (RHC = 0%). At RHA = 0% and humidified air, water transport by back diffusion from the cathode to the anode at the employed experimental conditions can support reasonable rehydration of the membrane and catalysts. At RHA = 0, a possible minimum RHC for considerable cell operation is temperature dependent. At RHC = 0 conditions, the cell can operate only at RHA = 100% with a loss that depends on T-cell. It was found that the internal cell resistance depends on RH, RHA, RHC and T-cell and it is consistent with the observed cell performance. (c) 2006 Elsevier B.V. All rights reserved.