The 607 nm complex of cytochrome c oxidase, formed aerobically in the presence of CO, appears as an intermediate during the oxidation of CO to CO2 by the enzyme. Maximal steady-state formation of this complex requires oxygen, high levels of carbon monoxide, and the presence of an endogenous hydrogen donor system or the addition of small amounts of reductant (both with isolated enzyme and mitochondrial preparations). The 607 nm complex can be formed after removing CO from the mixed-valence CO complex (cytochrome a3+a2(3)+CO) by aerating the presumably CO-free product. The elements of CO are, therefore, probably not part of the 607 nm complex nor of the related "compound C" produced at low temperatures.