The results of research of the properties of hybrid materials based on Nafion membranes, silica, phosphotungstic acid, and its acid salts (potassium, rubidium, and cesium) prepared by the in situ method has been described. Modification leads to an increase in the water content and a significant increase in the conductivity of the hybrid membranes, particularly at a low relative humidity. The materials containing slightly soluble acid salts of phosphotungstic heteropoly acid exhibit a lower conductivity than the material that contains the acid; however, the former are more stable. It has been found that the diffusion permeability of an HCl solution in Nafion + SiO2 + MxH3-xPW12O40 systems is lower than in Nafion + SiO2 and further decreases with increasing ionic radius of the cation. An explanation of the causes of these changes in the properties has been proposed.