Magnetic susceptibility measurements from 2 to 520 K, Mössbauer measurements from 1.2 to 450 K, and EPR measurements at 10 K have been performed on the monomeric FeIII complex (1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triaza-cyclononan)Fe. The complex exhibits a low-spin/high-spin transition at temperatures above 250 K. This behavior is quantitatively explained on the basis of a crystal-field model, which explicitly includes the vibrational properties of iron ligands. The EPR spectrum at 10 K yields a pure FeIII low-spin signal with g values 2.58(5), 2.12(5), 1.45(5). The values are consistently described by a crystal-field model, which explicitly includes spin-orbit coupling within the t2g subspace. The temperature dependence of the quadrupole splitting indicates a phase transition at approximately 100 K. The existence of the phase transition is corroborated by the temperature dependence of the effective thickness. The observation of only one quadrupole doublet up to 450 K indicates that the relaxation time between the high-spin and the low-spin configurations is shorter than the quadrupole precession time. X-ray structure analysis on single crystals at RT and temperature-dependent EXAFS investigation of powder material between 30 and 200 K indicate that the observed phase transition induces only changes of bond angles, while the low-spin/high-spin transition most likely induces changes of metal-ligand bond distances.