The heme-containing enzyme myeloperoxidase (MPO) is secreted from polymorphonuclear leukocytes and monocytes. It is involved in host defence and inflammation by oxidation of numerous small molecules. This review summarises our current results on the determination of redox properties of all intermediates involved in the halogenation and peroxidase cycle of MPO. The standard reduction potentials of the redox couples compound I/native MPO, compound I/compound II of MPO, and compound II/native MPO have been determined to be 1.16 V, 1.35 V, and 0.97 V, respectively, at pH 7 and 25 degrees C. Thus, for the first time, a full description of these important thermodynamic parameters of myeloperoxidase has been performed, allowing a better understanding of its extraordinary reactivity.