The high-spin → low-spin relaxation dynamics of the Fe(III) spin-crossover complexes [Fe(Sal₂tr)]PF₆ (H₂Sal₂tr = Bis(salicylaldimino)triethylenetetramine) and [Fe(acpa)₂]PF₆(Hacpa = N-(1-acetyl-2-propylidene)-2-pyridylmethylamine) are discussed within the theory of nonadiabatic multiphonon relaxation. A Huang−Rhys factor S of ≈25, estimated on the basis of average metal−ligand bond length differences Δr_HL of ≈ 0.12 Ã…, explains the observed low-temperature tunneling rate constants k_HL(T→0) of ≈ 10²Â s^-1 as well as the thermally activated process at T > ≈100 K semiquantitatively. The results obtained for the Fe(III) compounds are compared to those for Fe(II) spin-crossover compounds.