Abstract Equations are derived to predict the peak height, shape, and potential for the linear scan voltammetry of substrates that undergo mediated reduction at modified electrodes where the mechanism is complicated by preceding or following chemical reactions involving the immobilized couple. These coupled reactions occur whether or not the substrate is present. The primary effect of a preceding reaction is to shift the peak position on the potential axis; neither the shape nor height of the peak for the modified reduction of a substrate is altered. A following reaction influences the peak height and shape, but there is little effect on the peak potential. The treatment was applied to the investigation of the mechanism of reduction of hydrogen peroxide at a modified electrode.