Cyclic voltammetry has been used to study the unfolding dynamics of a redox-active heme protein, cytochrome b562 (cyt b562). Oxidized cyt b562 unfolds upon oxidation of folded, reduced cyt b562 in the presence of high denaturant concentrations. Voltammograms with rapid voltage scan rates show a reversible redox couple for the folded protein; the couple becomes irreversible at slower scan rates and a new wave due to unfolded protein appears. An unfolding rate of 0.038 s<sup>-1</sup> (2.2 M GuHCl, pH 7.0) was calculated from the scan-rate dependence of the disappearance of the peak corresponding to folded oxidized protein. Comparison of oxidized and reduced cyt b562 data suggests that equilibrium stability is a major determinant for folding speed.