The effects of the interfacial potential distribution on cyclic voltammetry of adsorbed redox molecules exhibiting slow (quasi-reversible and irreversible) electron transfer is considered in a theoretical model. Results of numerical simulations based on this model are presented and compared to cyclic voltammograms and measurements in the absence of double layer effects. The slopes of plots of peak potential versus log sweep rate are the same, i.e., RT/alpha nF and RT/(1 - alpha)nF for the cathodic and anodic reactions, respectively, as predicted in the absence of double-layer effects. The calculation of the rate constant can be greatly influenced by the double-layer effects.