A reversible temperature-dependent high-spin to low-spin transition with T 1/2 = −60 °C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme a 3 of the thermophilic ba 3 heme-copper oxidoreductase. The transition is based on the frequency shifts of the spin-state marker bands ν 2 (C b C b ) and ν 10 (C a C m ) and is attributed to the displacement of the heme iron along the heme normal as a consequence of the Fe-Np repulsion at temperature below −40 °C which will increase the ligand field strength forcing the pairing of d electrons into the lower energy orbitals. A reversible temperature-dependent high- to low-spin transition with T 1/2 = −60 °C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme a 3 of the thermophilic ba 3 heme-copper oxidoreductase.