Protein film voltammetry has been used to measure changes in the catalytic redox energetics of cytochrome c peroxidase produced by a single mutation in the distal pocket. Wild-type (WT) cytochrome c peroxidase adsorbs at a pyrolytic graphite edge electrode from ice-cold dilute succinate buffer, pH 5.4, to give an electroactive film showing a reversible and narrow (two-electron) signal, reduction potential 754 mV, which converts completely to a catalytic wave at a similar potential when low levels of hydrogen peroxide are added. Under the same conditions, the W51F mutant yields a weaker signal at 883 mV which also transforms to a catalytic wave at similar potential, but with amplitude comparable to that of WT. Zn either case the catalytic rates are very high. The reversible signals observed for each variant therefore correspond to the catalytic redox couple, analogous if not identical to Fe-IV=O,R+/Fe-III, with replacement of tryptophan-51 by phenylalanine causing a substantial increase in reduction potential (destabilization of Fe-IV=O,R+). The W51F variant appears less stable, even in the resting state, but this does not seriously undermine the results. When the two variants are studied in competition, the non-turnover voltammetry is dominated by the greater electroactive coverage of the WT enzyme, whereas peroxide reduction is controlled at all but the highest rotation rates by the more active W51F. The experiment provides a direct comparison of the real (thermodynamic) catalytic efficiencies of redox enzymes, in this case clearly identifying W51F as intrinsically the more active and efficient variant (higher reduction rates at lower driving force).