X-ray absorption near-edge structure (XANES) spectroscopy has been used to determine the oxidation state composition of the Mn site in Mn catalase under turnover conditions. The XANES data are consistent with parallel assignments based on electron paramagnetic resonance (EPR). However, a major advantage of the XANES assignments is that they permit the direct determination of the average oxidation states for derivatives that are EPR silent. In agreement with earlier work [Khangulov, S. V., Barynin, V. V., & Antonyuk-Barynina, S. V. (1990) Biochim. Biophys. Acta 1020, 25-33], these data show that the binuclear Mn site is reduced to Mn(II)/Mn(II) when peroxide is added in the presence of halide inhibitors. In addition, the present data provide the first direct evidence that the reduced enzyme is oxidized if peroxide is added in the absence of inhibitors. Under turnover conditions, the enzyme contains approximately a 2:1 ratio of Mn(II) and Mn(III). Similar results are obtained following incubation with dioxygen. These results are consistent with a Mn(II)/Mn(II)<==>Mn(III)/Mn(III) catalytic cycle and demonstrate that halide inhibition involves trapping the enzyme in the reduced state.