Strongly adsorbed species on an electrode surface are used to create a stable, redox-modified surface. Square wave voltammetry is then used to degrade the surface electrochemically, as evidenced by the resulting voltammetric response. This process can be mathematically modeled as a quasi-reversible surface reaction coupled with a first-order irreversible surface reaction of the product. This is the simplest possible model that can explain a two-step surface reduction. Exemplary calculations for square wave voltammetry show a wide variety of peak shapes depending on rate constants and square wave amplitude. The reduction of Dimethyl Yellow (4-(dimethylamino)azobenzene) adsorbed on mercury is accurately described by this model. Characteristic parameters of the overall surface process are obtained from voltammograms by using the two-step model with nonlinear least-squares analysis (COOL). For Dimethyl Yellow in Britton−Robinson buffer (pH 6.00) at a surface concentration of 17.3 pmol cm^-2, these parameters are as follows:  standard potential, E₁⁰ = −0.397 ± 0.001 V vs SCE; transfer coefficient for the first step, α₁ = 0.43 ± 0.02; rate constant for the first step, k₁⁰ = 103 ± 8 s^-1; transfer coefficient for the second step, α₂ = 0.11 ± 0.04; and rate constant for the second step, k₂⁰ (referenced to E₁⁰) = 11.1 ± 1.7 s^-1. Uncertainties are 95% confidence intervals derived from a pool of 11 voltammograms collected at different square wave amplitudes (E_sw = 0−100 mV).