Electro-osmotic drag and back diffusion are the primary water transport mechanisms in PEMFC (Proton Exchange Membrane Fuel Cell) electrolytes. These two phenomena occur competitively in the membrane, and ultimately determine the net water movement. The chemical compositions of reactants and product (i.e., H-2, O-2, and H2O) in a porous catalyst layer vary with respect to the electro-chemical reactions and water transport through the membrane. The tendency of the chemical compositions was estimated by analyzing the net water transport coefficient (alpha), defined as the ratio of the reaction rate to the water transport rate. New criteria were suggested for predicting species mole fractions from the flow direction, and these were validated by CFD analysis. The hydrogen mole fraction had different tendencies to rise or fall based oil the flow direction at alpha = 0.5, while the oxygen mole fraction extreme was located at alpha = -0.75. Finally, alpha was shown to influence the membrane conductivity and activation losses, which are the main factors that Contribute to fuel Cell performance. Published by Elsevier B.V.