In contrast to imidazolyl-phenoxyl radical models for the putative histidinyl-tyrosyl radical intermediate in the catalytic cycle of cytochrome c oxidase (CcO), imidazolyl-semiquinone radicals are stable and may be quantitatively produced. Internal electron transfer (valence tautomerism) within a Cu-II-imidazolyl-hydroquinonate species formed upon deprotonation of the parent Cu-II-imidazolyl-hydroquinone complex results in a substitutionally labile Cu-I-imidazolyl-semiquinone radical system. The results show the potential reactivity of a (transient) Cu-B(II)-histidinyl-tyrosyl anion intermediate in the catalytic cycle of CcO.