For PEMFC operation water management is very important to provide both sufficient proton conductivity and mass transport Therefore in this study the effect of the relative humidity (38-87%) on cell performance is examined for PEMFC MEAs with various Nafion ionomer contents The MEAs were fabricated using a CCM (catalyst coated membrane) spraying method As the relative humidity of the cathodes (RHC) increases the cell volt ages at 04 and 1 2 A/cm(2) increased for a MEA with 20 wt% ionomer This can be explained in terms of the expansion of active sites with enhanced ionic conductivity (activation overpotential) In contrast with a higher RHC value the cell voltages for 35 wt% ionomer (more hydrophilic) gradually decreased as a result of slower gas transport (concentration overpotential) For MEAs with intermediate ionomer contents 25 and 30 wt%, the cell voltages at 04 A/cm2 showed maximum values at a RHC of 67% at which point the mass transport begins to be the more dominant factor The highest unit cell performance was observed in a MEA with an ionomer content of 25 wt% at a RHC of 59% and in a MEA at 20 wt% ionomer content at a higher humidity of 87%.