Proton-conducting electrolytes based on a sulfonated hydrogenated poly(butadiene-styrene) matrix were synthesized. Block copolymer ionomers were prepared through sulfonation of part of the styrene units. The free acid samples were characterized with X-ray photoelectron spectroscopy to quantify sulfonation. Microstructural and electrical characterizations of membranes obtained from sulfonated polymer and blends were carried out with dynamic mechanical analysis, differential scanning calorimetry, and impedance spectroscopy. Electrolytes exhibiting the highest conductivities were tested in a model hydrogen/oxygen single fuel cell working at a medium temperature range (30-80 degreesC) and at 1 bar of pressure. The current and power densities obtained were compared with commercial Nafion electrolyte.