The theoretical analysis of mass-transport phenomena at the cathode in proton exchange membrane fuel cells is a consequence of the experimental analysis, reported in Part I of this paper. A one-dimensional model for the substrate-gas diffusion and active layers was assumed to elucidate the contributions to mass-transport overpotentials. The results of the theoretical analysis show that, first, the higher slope of the pseudo-linear region of the potential (E) vs. current density (i) plot with air or the gas mixtures (O2/He, O2/Ar, O2/N2), rather than with O2, as the cathodic reactant, is predominantly due to mass-transport limitations in the active layer; and, second, the departure from linearity of the E vs. i plot is due to mass transport in the substrate-diffusion layer. Such mass-transport phenomena are considerably less with O2/He than with O2/Ar or O2/N2 because of the fact that He is a considerably lighter and smaller molecule, and the mass-transport parameters for oxygen diffusion are more favorable with O2/He than with O2/Ar or O2/N2 gas mixtures. The mass-transport limitations in the substrate-gas diffusion layer are probably caused by water droplets or films in this layer. By altering the Teflon content, porosity and/or thickness of this layer, mass-transport overpotentials in this layer could be considerably decreased. By increasing the oxygen concentration in the gas mixtures to above 40%, mass-transport limitations are significantly reduced. Alternatively, increase of pressure from 1 to 3 or 5 atm, has similar effects. At 1 atm pressure, the expected increase in performance with temperature is not observed because of the anomalous effect of decrease of oxygen partial pressure and enhanced electrode kinetics.