An analytical expression was derived for the frequency dependence of the potential-modulated UV-visible reflectance (electroreflectance : ER) spectroscopic signal for the redox reaction of solution-phase species based on an equivalent circuit of the electrode \ solution interface. A basic equation was also given for the derivation of the ER signal due to the redox reaction of either solution-phase or surface-confined species. The result was examined experimentally in the ER measurements of the redox reactions of methylviologen at a highly-oriented pyrolytic graphite electrode and hexacyanoferrate at a gold electrode in the aqueous solutions. The ER data obtained showed a good agreement with the prediction by the theory.