The effect of modification with hydrated silica on the ion selectivity, proton conductivity, and electroosmotic permeability of the Nafion 115 membrane has been experimentally studied. The general trends of change in the electrotransport characteristics of perfluorinated membranes in hydrochloric acid and sodium chloride solutions have been revealed. The "true" ion transport numbers have been theoretically determined using the parameters of the three-wire model, which characterizes the mechanism of electric current flow across a swollen ion-exchange membrane, and taking into account the electroosmotic transport of water in accordance with the Scatchard equation.