Parabanic acid reduction has been studied in the -1.4 less than or equal to H-0 less than or equal to 0.9 acidity range by the ac impedance technique. Changes in the adsorption of reactant and products with solution acidity lead to protonation equilibrium constants that differ from their bulk values. The observed pK(a) shifts were consistent with interfacial dielectric saturation. Variation of the charge transfer parameters with potential and proton activity allows a detailed analysis of the reduction mechanism to be performed in terms of the scheme of squares.