Cytochrome c1aa3 from Thermus thermophilus has optical and EPR properties similar to bovine cytochrome c oxidase. We have studied 87Fe-enriched samples with Mössbauer spectroscopy in the fully oxidized and fully reduced states and in the oxidized state complexed with cyanide. The cytochromes a and c1 yielded spectra quite similar to those reported for the cytochromes c and b5; in the oxidized state the spectra reflect noninteracting, low spin ferric hemes, whereas the a- and c1-sites of the reduced enzyme are typical of low spin ferrous hemochromes. The spectra of the reduced enzyme show that reduced cytochrome a3 is high spin ferrous, with Mössbauer parameters quite similar to those of deoxymyoglobin. Upon addition of cyanide to the oxidized enzyme, the a3-site exhibits in the absence of an applied magnetic field and at temperatures down to 1.3 K a quadrupole doublet with parameters typical of low spin ferric heme-CN complexes. The low temperature spectra taken in applied magnetic fields show that the electronic ground state of the a3-CN complex has integer electronic spin, suggesting ferromagnetic coupling of the low spin ferric heme (S = 1/2) to Cu2+ (S = 1/2) to yield as S = 1 ground state. We have examined the oxidized enzyme from two different preparations. Both had good activity and identical optical and EPR spectra. The Mössbauer spectra, however, revealed that the a3-site had a substantially different electronic structure in the two preparations. Neither configuration had properties in accord with the widely accepted spin-coupling model proposed for the bovine enzyme.