Sulfonated poly(etheretherketone) (SPEEK), with 577 g per equivalent proton, was made by the direct sulfonation method using sulfuric acid. Composite membranes were prepared by the addition of 0, 5, 15, 30, and 40 wt % phosphotungstic acid (PTA) to the nonwater soluble SPEEK. After Cs+ stabilization of the solid acid-containing composites was done, the membranes' conductivities were tested as alternatives to Nafion in proton exchange membrane fuel cells. The presence of Cs-PTA in the membranes had little effect on conductivity at low relative humidity (RH), although a 30-40% loading did enable sigma = 0.12 S/cm at 80 degrees C and 100% RH. A characterization of Cs-PTA and composite membranes via thermogravimetric and differential thermal analysis, X-ray photoelectron spectroscopy, Fourier transform infrared, and water uptake measurements revealed that stabilization of the PTA with a Cs+ ion also resulted in a sharply curtailed ability to uptake and hold water, a necessary attribute of a significant protonic mobility. Even so, the presence of Cs-PTA in SPEEK membranes enabled improved current-voltage curves in a fuel cell configuration by reducing the interfacial resistance between SPEEK and the Nafion-based catalyst particle binder. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3425618] All rights reserved.