The preparation of new, cation-exchange membranes from polymer composites based on poly(vinylidene fluoride), sulfonated polystyrene-co-divinylbenzene, and antimonic acid is. reported. The thermal properties of the composites have been characterized by differential scanning calorimetry Values of the transport number of protons in the membranes were obtained from the observable electric potential. It is defined from the potential difference measured between the electrodes reversible to one of the constituent ions in equilibrium with the system. When compared with Nafion cation-exchange membranes, the membranes described in this work exhibit interesting proton transport properties that could make them suitable as polymer electrolytes in fuel cells.