CuA, an electron transfer center present in cytochrome c oxidase, COX, and nitrous oxide reductase, N2OR, is a dimeric copper complex with four ligands, two cysteine thiols bridging the metal ions and two terminal histidine residues. The center cycles between the mixed-valence state [Cu(I),Cu(II)] and the reduced state [Cu(I),-Cu(I)]. The EPR, optical absorption, low-temperature magnetic circular dichroism, and CD spectra of three proteins containing the mixed-valence state of CuA have been measured between 33 000 and 5000 cm^-1. These results point to two forms of the chromophore, one in the enzyme N2OR of Pseudomonas stutzeri, lacking its catalytic center, and also in a water soluble domain of subunit II of Paracoccus denitrificans COX and the other, referred to as CuA*, in a site engineered into a soluble domain of subunit II of the quinol oxidase in Escherichia coli. An assignment of the electronic spectrum has been made in terms of a covalent planar core [Cu2(SR)2]⁺ with a Cu-S distance of 2.2 Å, a Cu-Cu distance of 2.5 Å, and a Cu-S-Cu angle of 70°. Molecular orbitals arising from five 3d orbitals on each copper and two lone-pair thiolate orbitals on each cysteine ligand divide into three sets, four bonding (with respect to the Cu-S interaction) orbitals at lower energy, four antibonding orbitals at higher energy, and six intermediate nonbonding orbitals. The inversion center of the copper core imposes rather strict selection rules giving rise to two pairs of allowed electronic transitions, polarized along either the S-S or the Cu-Cu axis. A delocalization energy of ~4500 cm^-1 can be estimated from the data, which is at least 1 order of magnitude larger than the vibrational energies of the core, accounting for the stability of the class III or delocalized mixed-valence form. The CuA sites in COX and N2OR have essentially identical electronic structures with complete delocalization. However, the CuA* site shows partial trapping of the valences. The close approach, to ~2.3 Å, of the backbone carbonyl group of a conserved glutamic acid residue is proposed to be responsible for this partial localization. CuA is a highly covalent planar rhomb which provides an effective path, with low reorganization energy, for electron transfer across subunit II from cytochrome c to the cytochromes a and possibly a₃ of COX.