A model of a polymer electrolyte membrane fuel cell (PEMFC) is proposed to predict the polarization curves and corresponding impedance diagrams for a small membrane electrode assembly sandwiched between bipolar plates. This theoretical approach takes into account mass transport for the following species: reactants and products along the flow channel, and water across the membrane. A global mass transport coefficient allows characterization of the mass transport flux through the gas-backing layer of the GDE, and the flooded agglomerate model is used to describe the active layer within the gas diffusion electrode. Applying this model to usual PEMFC shows that diffusion plays a crucial role at cathode for high current densities when using air as reactant. Finally, the influence of working conditions and PEMFC structure is investigated.