ABSTRACT A study has been made of the oxidation-reduction of manganese and iron hematoporphyrin IX and etioporphyrin I. The absorption spectra of the +2 and +3 valence states are reported for both complexes. For the manganese compounds three valence states are reported, namely, the +2, the +3 and the +4. The complete spectra of the manganese hematoporphyrin IX in all three valence states are given. The redox potentials for the manganese hematoporphyrin in a water solution have been determined as a function of pH, and it is shown that the MnII to MnIII couple is at about the molecular hydrogen potential at neutral pH, while the MnIII to MnIV couple is higher than the oxygen potential at neutral pH. It thus appears that a photochemical transformation of MnII to MnIV with a suitable electron acceptor could provide the oxidation potential required to liberate oxygen from water.