The effect of ion exchange capacity (IEC) of tetrafluoroethylene-g-polystyrene sulfonic acid membranes (ETFE-g-PSSA) on the electrochemical oxygen reduction reaction at the membrane|catalyst layer interface is investigated. In the half-fuel cell system the catalyst layer is hot-pressed to an ETFE-g-PSSA membrane forming a half-membrane electrode assembly (HMEA). The working electrode is exposed to liquid electrolyte at the membrane side and to 02 gas at the gas diffusion layer side. Measurement of electrochemically active surface area by CO stripping reveals that membranes with higher IECs result in higher active surface areas of the electrode. In contrast, they exhibit lower oxygen reduction performances. The experimental results are analyzed using the agglomerate model. The extracted effective diffusion coefficients of oxygen (0.62-1.25 x 10(-5) cm(2) s(-1)) and O-2 solubility (2.03-2.80 mmol l(-1)) in the catalyst layers are close to the corresponding values found in fully hydrated ETFE-g-PSSA bulk membranes and H2SO4. Percolation theory reveals that similar to71-77% of void space in the catalyst layer is filled with water. The membrane IEC regulates the extent of flooding in the cathode, which in turn affects its electrochemical characteristics. The predominant mode of 02 transport in the half-fuel cell is diffusion through the electrolyte phase or flooded area. (C) 2004 Elsevier B.V. All rights reserved.