Mixtures of cytochrome c oxidase and cytochrome c have been titrated by coulometrically generated reductant, methyl viologen radical cation, and physiological oxidant, O 2. Charge distribution among the heme components in mixtures of these two redox enzymes has been evaluated by monitoring the absorbance changes at 605 and 550 nm. Differences in the pathway of the electron transfer process during a reduction cycle as compared to an oxidation cycle are indicated by variations found in the absorbance behavior of the heme components during successive reductive and oxidative titrations. It is apparent that the potential of the cytochrome a heme is dependent upon whether oxidation or reduction is occurring.