This paper reports further studies to understand and optimize the Membrane and Electrode Assembly (MEA) structure in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). The effective proton conductivity in the active catalyst layer was measured as a function of its composition, which consisted of platinum catalyst on carbon support (E-Tek) and Nafion(R) polymer electrolyte (DuPont de Nemours). The conductivity was calculated from the resistance added to a standard MEA by the addition of an inactive composite layer in the electrolyte path between the anode and cathode. The specific conductivity of the active layer was found to be proportional to the volume fraction of Nafion(R) in the composite mixture, following the relationship kappa(H+)(eff) approximate to 0.078 epsilon(Nafion) + 0.004 S cm(-1). Modeling studies showed that this ionic conductivity limits the utilized active layer thickness to 20-25 mu m