In this work, an experimental study of the impedance response of H-2/O-2 polymer electrolyte membrane fuel cells (PEMFC) was carried out with single cells with four Nafion(R) membranes (117, 115, 1135 and 112) of different thicknesses, at four temperatures in the range 25-80 degreesC, with reactant gases humidified under different conditions. The impedance results were analysed in terms of the high frequency resistance, the contribution of the charge transfer due to the oxygen reduction reaction and a low frequency relaxation process. With respect to the system, the results are analysed in terms of the membrane thickness and history, the working temperature of the cell and the humidification conditions of the reactant gases, and show clearly that thinner membranes present better characteristics of water management, being much less sensitive to temperature and current density changes.