A membrane-electrode assembly for a mixed reactant fuel cell system is provided. The membrane-electrode assembly does not require a separator that physically separates the membrane-electrode assemblies from each other in a stack. The membrane-electrode assembly of the present invention instead includes an electrode substrate that is disposed on a surface of an anode or a cathode of the membrane-electrode assembly. The electrode substrate has a flow path, through which a fuel and an oxidant are supplied. The fuel and oxidant are absorbed into the electrode substrate and further into the anode and the cathode. The fuel and the oxidant are selectively oxidized and reduced in the anode and the cathode, respectively, to produce electricity.