The simultaneous effect of two substituents on the redox potentials, E-1/2'S, of N, N'-di (p -substituted benzyl)viologens (1-X,Y) and of 4-(p-substituted benzoyl)-N-(p-substituted benzyl) pyridinium cations (2-X,Y), was investigated in terms of the substituent constants, sigma(p-X) and sigma(p-Y). Systems 1-X,Y and 2-X,Y were chosen because they both undergo successive outer sphere one electron (1-e) reductions, and therefore there is no ambiguity regarding the mechanism or the rate-determining step. Substituents (X and Y) were chosen among the -NO2, -Br, and -OCH3 groups in six different combinations for 1-X,Y's and nine different ones for 2-X,Y's. Response surfaces are essentially three-dimensional Hammett plots. Statistical analysis according to a quadratic model derived from the Taylor series expansion of the general extrathermodynamic relationship E-1/2 = F(sigma(p-X),sigma(p-Y)) that describes the dependence of E-1/2'S on the substituent constants shows that the substitution effect is not additive. Quite strong nonlinear terms appear when substituents can interact through resonance with a site involved in the electron-transfer process. This, for example, is the case for the second -e reduction of 2-X,Y's. Other significant nonlinear effects are attributed to stereoelectronic effects in combination with the high polarizability of pi-electrons. Our results have important implications on our ability to predict the effect of tuning of the redox potential of redox-active species through substitution.