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 ( , , ), rather than with , 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. plot is due to mass transport in the substrate‐diffusion layer. Such mass‐transport phenomena are considerably less with than with or 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 than with or 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.