Chlorite dismutases (Clds) are heme b containing oxidoreductases that convert chlorite to chloride and molecular oxygen. In order to elucidate the role of conserved heme cavity residues in the catalysis of this reaction comprehensive mutational and biochemical analyses of Cld from “Candidatus Nitrospira defluvii” (NdCld) were performed, using UV-vis spectroscopy, spectroelectrochemistry, pre-steady-state and steady-state kinetics as well as X-ray crystallography. The results obteined are described.