Reduction potentials for the catalytic compound I/compound II and compound II/Fe3+ redox couples, and for the two-electron compound I/Fe 3+ redox couple, have been determined for ascorbate peroxidase (APX) and for a number of site-directed variants. For the wild type enzyme, the values are E°′(compound I/compound II) = 1156 mV, E°′(compound II/Fe3+) = 752 mV, and E°′(compound I/Fe3+) = 954 mV. For the variants, the analysis also includes determination of Fe 3+/Fe2+ potentials which were used to calculate (experimentally inaccessible) E°′(compound II/Fe3+) potentials. The data provide a number of new insights into APX catalysis. The measured values for E°′(compound I/compound II) and E°′-(compound II/Fe3+) for the wild type protein account for the much higher oxidative reactivity of compound I compared to compound II, and this correlation holds for a number of other active site and substrate binding variants of APX. The high reduction potential for compound I also accounts for the known thermodynamic instability of this intermediate, and it is proposed that this instability can account for the deviations from standard Michaelis kinetics observed for most APX enzymes during steady-state oxidation of ascorbate. This study provides the first systematic evaluation of the redox properties of any ascorbate peroxidase using a number of methods, and the data provide an experimental and theoretical framework for accurate determination of the redox properties of Fe3+, compound I, and compound II species in related enzymes. © 2007 American Chemical Society.