The effect of ion exchange capacity (IEC) of polymer electrolyte membranes (PEMs) on cathode catalyst layer operation is investigated using a hydrogen/oxygen proton exchange membrane fuel cell (PEMFC) and a series of tetrafluoroethylene-g-polystyrene sulfonic acid (ETFE-g-PSSA) membranes. The electrochemically active surface area (ESA) of the catalyst layer reveals a slight dependence on IEC. The steady-state beginning-of-life polarization curves show an increase in fuel cell performance with increased IEC. The membrane's IEC and molecular structure controls the water content within, and regulates the water balance in the complete MEA. Comparing half-fuel-cell and fuel cell systems reveals that the ESA in the latter is lower as a result of reduced wetting of the catalyst layer but this is offset by an order of magnitude improvement of the effective O-2 diffusion. Consequently oxygen reduction reaction (ORR) performance is higher in the fuel cell system. The balance between electro-osmotic flux and hydraulic counterflux in the membrane is employed to explain the distinct effects of IEC in the half fuel cell and fuel cell systems. The two types of measurements thus provide a convenient tool for studying the interplay of different mechanisms of water flux in the membrane. (c) 2005 The Electrochemical Society.