The cathode compartment of membrane electrode assembly (MEA) of the proton-exchange-membrane fuel cell (PEMFC) is studied theoretically. A pseudo-phase-equilibrium approach adopting an approximate phase-equilibrium equation, incorporated with the associated equations of gaseous multi-component diffusion, liquid water capillary transport, and surface electrochemical kinetics, well characterizes the performance of cathode electrode under unsaturated feed. The pseudo-phase-equilibrium approach avoids the need of explicit liquid water front tracking so that only a single domain formulation, without consideration of the interior boundary, is required for the simulation. In order to illustrate the capability of the proposed approach, a mathematical model of the cathode compartment of MEA in one-dimension is formulated, in which gas species concentration profiles, liquid water distribution, and liquid water front are calculated. The validity of the pseudo-phase-equilibrium approach is then evaluated over an extensive polarization range under specified operating temperature, pressure, and inlet humidity. The solutions obtained using the pseudo-phase-equilibrium approach and the exact phase-equilibrium equation are compared over a wide range of parameter values. In addition, the influences of important transport parameters such as water transport coefficient, gas diffuser porosity, and absolute liquid permeability are evaluated and discussed. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.