Well-resolved Soret-excited resonance Raman spectra have been obtained in the heme marker band region (1440-1700 cm-1) for the fully oxidized and fully reduced forms of the terminal oxidase aa3 from Sulfolobus acidocaldarius. The results demonstrate that in both heme groups the structural properties depart from those of other aa3 oxidases. In the fully oxidized form, the formyl stretching vibration of heme a is observed at 1656 cm-1, approximately 7 cm-1 higher in frequency than found for beef heart cytochrome c oxidase. The frequency of this vibration shows a slight upshift to 1657 cm-1 in the fully reduced form. Thus, the formyl group of the heme a seems not to be involved in a significant hydrogen bond, in sharp contrast to both beef heart cytochrome c oxidase and other quinol-oxidizing enzymes. For heme a3, two conformers were detected in the fully oxidized state. The observation of two v3 modes at 1482 and 1490 cm-1 indicate the coexistence of both the normal six-coordinated high spin and a new five-coordinated high spin configuration. Both heme a3 species exhibit different formyl stretching vibrations at 1673 and 1666 cm-1, respectively. In the fully reduced enzyme, the identification of two heme a3 conformers is not unambiguous.