By chemical deposition of silver in an ion-exchange matrix, the Ag-0/MF-4SK/C nanocomposite has been fabricated, which is capable of changing the amount of deposited silver by varying the concentration of ionic groups in the MF-4SK membrane. Transmission electron microscopy has revealed the presence of silver particles of a 3 nm size. On an electrode coated with the MF-4SK membrane, electroreduction of oxygen occurs on the carbon substrate and the membrane is a diffusion barrier. Since the polymer matrix contains counterions H+ localized near fixed -SO3 groups, the hydrogen ion takes part in the stage of charge transfer to the oxygen molecule. An increase in the dioxygen electroreduction current on the Ag-0/MF-4SK/C nanocomposite relative to the MF-4SK/C composite is due to the presence of silver particles, which result in acceleration of the reaction by the catalytic action, and to an increased contribution of the four-electron process as compared with the two-electron process characteristic of carbon materials.