The conductivity of mixtures of phosphoric acid with poly(ethylene imine) has been studied, and it was found that the conductivity of such mixtures with high acid content can be enhanced by the addition of highly dispersed silica (fumed silica). At the same time, silica addition increases the stiffness of the polymer, and macroscopically solid composites with good proton conductivity can be obtained, without significant degradation of the optical transparency of the polymer electrolyte.