1. 1. A comparison is made between the oxidizing agents found to convert copper phthalocyanine to a higher oxidation state in which an electron is probably removed from a π-orbital, and those agents which react with horse-radish peroxidase giving intermediate compounds having the same spectroscopic characteristics as the mixture of compounds I and II formed from H 2O 2, a system where compound II is also a single-equivalent oxidation product. The evidence suggests that in both cases the higher oxidation states arise by electron or hydrogen atom removal. 2. 2. Hemin under the same conditions used in the phthalocyanine experiments undergoes an irreversible oxidation, there is no evidence for a similar single-equivalent oxidation intermediate although it may be formed and react too rapidly to be observed. 3. 3. The change in absorption spectra when the copper phthalocyanine undergoes one-electron oxidation bears no resemblance to the change when peroxidase or ferrimyoglobin are converted to their one-equivalent higher oxidation states; for this reason and the observed hydrogen-ion dependence in the reduction of the hemoprotein compounds, it is suggested that a π-orbital structure is not present in these compounds.