The effect of carbon loading in the microporous layer on the PEM fuel cell performance was investigated using mercury porosimetry, electrochemical polarization techniques and electrochemical impedance spectroscopy. When air was used as an oxidant, a maximum fuel cell performance was obtained for a carbon loading of 0.5 mg cm(-2). The ac-impedance properties were analyzed based on the thin film/flooded-agglomerate dynamics in the catalyst layer and the oxygen diffusion kinetics in the gas diffusion layer. The ac-impedance study indicated that the optimized microporous layer results in an effective water management (i.e. a balancing of water saturations in the catalyst layer and the gas diffusion layer), thereby improving the oxygen diffusion kinetics. (c) 2006 Elsevier B.V. All rights reserved.