A study of the near-infrared absorption spectra of three oxygen compounds of membrane-bound cytochrome oxidase (ferrocytochrome c:oxygen oxidoreductase; EC 1.9.3.1) shows that the formation of compound A (oxycytochrome oxidase) causes no significant infrared absorbance changes at -103 degrees. At -64 degrees, the formation of compound C from the mixed-valence state of the oxidase leads to increased absorption at 740-750 nm. The formation of compound B at -84 degrees from the fully reduced state of the oxidase causes increased absorption at 790-800 nm. Further oxidation of cytochrome oxidase results in increased infrared absorption at 820-830 nm at -60 degrees. The position of the infrared absorption band in compound C thus depends at least upon the oxidation-reduction state of heme a and its associated copper atom. Compound C contains two types of oxidized (cupric) copper; that associated with heme a is initially oxidized, and that associated with heme a3 is oxidized as a second step in the reaction with oxygen. Compound C exhibits a unique intense absorption band at 606-609 nm that is tentatively assigned to a charge transfer interaction between heme a3 in the reduced state and its associated copper in the oxidized state, with heme a and its associated copper in the oxidized state.