Abstract This paper describes the use of a novel magnetic circular dichroism-microwave double resonance (MCDODMR) experiment to study the optical properties of the EPR detectable copper center, Cu A 2+, in bovine cytochrome c oxidase. By irradiating the sample with a monochromatic microwave beam of appropriate frequency it is possible to quench partially the MCD intensity of the features due to Cu A 2+. In this way the MCD bands from this center have been identified even in the presence of overlapping optical transitions from the haem centers of this enzyme. The resulting spectrum compares well with that reported some years ago from this laboratory and obtained by measuring MCD magnetisation characteristics. In addition the shapes of the MCD-ODMR lines obtained in a plot of MCD intensity against magnetic field strength have been analyzed to yield the relative polarizations of the optical transitions of Cu A 2+ which contribute to the MCD spectrum. All of the bands observed between 450 and 850 nm are predominantly polarized in the xy plane perpendicular to the direction of the g-tensor component of Cu A 2+ at g = 2.18. This suggests that all of the Cu A 2+ ligands that contribute to the optical charge-transfer transitions in the visible region lie approximately in the basal plane. Possible structures for Cu A 2+ can now be suggested.