A two dimensional, non-isothermal mathematical model for the entire sandwich of a proton exchange membrane (PEM) fuel cell including the gas channels is developed. To take into consideration the real concentration distributions along the interface between the gas diffuser and catalyst layer, transport equations are solved simultaneously for the domain consisting of the coupled gas channel, gas diffuser, catalyst layer and membrane. The selfconsistent schematical model for porous media is used for the equations describing transport phenomena in the membrane, catalyst layers and gas diffusers, while standard Navier-Stokes, energy transport, continuity and species concentration equations are solved in the gas channels. The oxygen mole fraction distribution in the coupled cathode gas channel - gas diffuser is studied. Influences of the inlet conditions at the gas channel entries and of the gas diffuser porosity on the cell performance are also analyzed.