Silver tripolyphosphate films were formed on slightly cathodically polarized silver electrodes in aqueous solutions containing Ag⁺ and tripolyphosphate ions. The solutions were kept supersaturated with silver tripolyphosphate during the film formation. The potential of the silver electrode, on which the silver tripolyphosphate film was formed at a very low current density, showed a linear relationship to the tripolyphosphate ion concentration between 10^-4 M and 10^-1 M.