For high efficiency and long durability of proton exchange membrane fuel cells (PEMFCs), polymer electrolyte membranes should be kept wet. Reactant gases should be humidified on this account. For the humidification, the PEMFC system uses an external or internal humidifier as a part of balance of plants (BOPs). However, external humidifiers have many disadvantages such as parasitic power loss, system complexity, high cost and bulky volume. As such, efforts have been made to remove the external humidifier or replace it with an advanced humidifier. In this work, to remove a humidifier, humidification by exhaust gas recirculation is investigated by theoretical analysis and experiments with 5-cell stack of an active area 250 cm(2). In the theoretical analysis, species conservation equations and energy conservation equation are solved to obtain the O-2 concentration, stoichiometric ratio, humidity ratio, temperature, amount of condensed water and so on. With the theoretical results, experiments with 5-cell, 250 cm(2) fuel cell stack were carried out in order to analyze the stack performance at the theoretical conditions of the cathode process stream of exhaust gas recirculation. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.