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 a₃ of the thermophilic ba₃ heme–copper oxidoreductase. The transition is based on the frequency shifts of the spin-state marker bands ν₂ (C_bC_b) and ν₁₀ (C_aC_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.