In the visible/NIR (600-900 nm) three different redox centres are potentially detectable in vivo in mitochondrial cytochrome c oxidase: haem a (605nm), the binuclear haem a3/CuB centre (655 nm) and CuA (830 nm). In this paper we report changes in the steady state reduction of these centres following increases in the rate of electron entry into the purified enzyme complex under conditions of saturating oxygen tension. As turnover is increased all three centres becomes progressively reduced. Analysis of the steady states indicated that all three centres remained in apparent equilibrium with cytochrome c throughout the titration. The calculated redox potentials of CuA (+224 mV) and haem a (+267 mV) were consistent with previous equilibrium data. The 655 nm band was also found to be oxygen and flux sensitive. It may be a useful additional in vivo detectable chromophore. However, it titrated with an apparent redox potential of +230mV, far from its equilibrium value (+400 mV). The implications of these results for the interpretation of non invasive measurements of mitochondrial function are discussed.