A theory is presented to account for the characteristics of cyclic voltammograms (CVs) at monolayer covered electrodes. It is shown that CVs of reversible systems can exhibit peak separations of greater than 57/n mV, a characteristic previously associated with quasi-reversible systems. In the presence of a monolayer, the dimensionless current function is less than the value for an ideal reversible reaction at a bare electrode. The theory was tested experimentally for the reduction of cytochrome c at a 4,4’-bipyridal disulfide-modified gold electrode. A good correspondence was obtained between predicted (68 mV) and experimental (69 +/- 4 mV) peak separations. The value of the diffusion coefficient of cytochrome c calculated from a plot of current vs sweep rate was 10.2 x 10(-7) cm(2) s(-1), which is in good agreement with literature values. In contrast, using the traditional CV model for a bare electrode, the diffusion coefficient was calculated as 8.8 x 10(-7) cm(2) s(-1).