The effect of surface heterogeneity in dc and ac voltammetries has been investigated with a variety of dual-electrode materials and electrode reactions for quasireversible electron-transfer reactions in which the substrate and product are present as solutes. Typically, and as theoretically expected, when two distinctly different surfaces are present, the faradaic currents arising from each region combine additively. This leads to a marked effect on the shape of the transient response, particularly in the ac voltarnmetric case. However, heterogeneity is not possible to detect when the rate constants for the two regions are identical in magnitude (or reversible). At the other extremity where very large differences in rate constants occur, it is possible to detect fully resolved voltarnmetric process. For example, the presence of a small region of edge planes on an otherwise highly ordered pyrolytic graphite electrode surfaces clearly reveals itself in the case of [Fe(CN)(6)](3-) electro-reduction. In this example, the vastly enhanced reversibility of the electron-transfer at defect sites present in the edge planes, compared with the much slower reaction on the highly ordered pyrolytic graphite surface leads to detection of two well resolved [Fe(CN)(6)](3-/4-) processes from an experiment at a single electrode. (c) 2008 Elsevier B.V. All rights reserved.