Composite polymer membranes are prepared by embedding layered silicates such as Laponite and Montmorillonite (MMT) into sulfonated poly(ether ehter ketone) (sPEEK) membranes for fuel-cell applications. Sulfonation of the polymer increased membrane hydrophilicity to give good proton conductivity. Layered silicates incorporated into polymer membranes help to reduce swelling significantly in hot water; they also help to decrease methanol permeability. These polymer/clay composite membranes show thermal stability to 240 degreesC and (3-3.5) x 10(-3) S cm(-1) proton conductivity at room temperature. In addition, methanol cross-over is reduced without a serious reduction in the proton conductivity. In a single-cell test using hydrogen and oxygen, the prepared membranes give current densities that are between 70 and 80% of those with Nafion 115 membranes. As a result, for polymer electrolytes, sPEEK/clay composite membranes offer a low-cost alternative to perfluorinated membranes. (C) 2003 Elsevier B.V. All rights reserved.