The double layer capacitance at the interface between a cured epoxy resin and a metal was measured by cyclic voltammetry, and the a.c. technique was applied in order to access the impedance data as a measure of adhesion to the metal. Since epoxy resin is a highly insulating material, it was ionized by partially introducing a sulfonate group into the epoxy ring for electrochemical measurements. The other part of the epoxy ring was retained for making the network polymerization. The sulfonated epoxy resin cured on an interdigitated microelectrode or a parallel plate electrode pair showed reproducible voltammograms, in which current was proportional to sweep rate. Capacitance was obtained from the a.c. impedance analysis at various temperatures, and was found to be almost constant up to the glass transition temperature. In contrast, the epoxy resin mixed with Na2SO3 showed temperature dependence of the capacitance of Arrhenius type. When the surface was contaminated with alumina powder, its capacitance decreased drastically.