Circular dichroism spectra of bovine heart aa(3)-type cytochrome c oxidase have been studied with a major focus on the Soret band π → π* transitions, B(0(x,y)), in the two iron porphyrin groups of the enzyme. The spectra of the fully reduced and fully oxidized enzyme as well as of its carbon monoxide and cyanide complexes have been explored. In addition, CD spectra of the reduced and oxidized ba(3)-type cytochrome c oxidase from Thermus thermophilus were recorded for comparison. An attempt is made to interpret the CD spectra of cytochrome c oxidase with the aid of a classical model of dipole-dipole coupled oscillators taking advantage of the known 3D crystal structure of the enzyme. Simultaneous modeling of the CD and absorption spectra shows that in the bovine oxidase, the dipole-dipole interactions between the hemes a and a(3), although contributing significantly, cannot account either for the lineshape or the magnitude of the experimental spectra. However, adding the interactions of the hemes with 22 aromatic amino acid residues located within 12 Å from either of the two heme groups can be used to model the CD curves for the fully reduced and fully oxidized oxidase with reasonable accuracy. Interaction of the hemes with the peptide bond transition dipoles is found to be insignificant. The modeling indicates that the CD spectra of cytochrome oxidase in both the reduced and oxidized states are influenced significantly by interaction with Tyr244 in the oxygen-reducing center of the enzyme. Hence, CD spectroscopy may provide a useful tool for monitoring the redox/ionization state of this residue. The modeling confirms wide energy splitting of the orthogonal B(x) and B(y) transitions in the porphyrin ring of heme a.