Sulfonation of PEEK-WC powder (a glassy poly-ether ether ketone) has been performed at 80 °C for 3 h using H2SO4 conc. (96%). The degree of sulfonation varied from 15 to 40%, depending on the reaction time. The sulfonated polymers (S-PEEK-WC) were used to prepare dense membranes for possible fuel cell application. In particular, two membranes were prepared by solvent evaporation technique, using DMA as the solvent for the 2- and 3-h sulfonated polymers. The dense character of both membranes has been confirmed by SEM analysis. The chemical–physical characterization has showed an increasing water solubility of the sulfonated polymers with the increasing level of sulfonation: from 7 to 11% of weight loss at 80 °C in pure water. Despite this, a water uptake increase up to 15% has been found for the dense membranes at 80 °C. Furthermore, if compared with a commercial Nafion membrane, S-PEEK-WC membranes exhibit comparable electrochemical performances even if at higher temperature, especially the membrane prepared with the 3-h sulfonated polymer. A power density of 284 mW/cm2 was obtained for S-PEEK-WC membrane at 120 °C in H2/air fuel cell, slightly above the corresponding value found for Nafion. At the same temperature, a Tafel slope of 70 mV was reached for the S-PEEK-WC membrane versus 100 mV for the Nafion. Finally, the specific conductivity is about 1.7 × 10−2 S/cm versus 1.4 × 10−1 S/cm of the commercial Nafion membrane.