A study of the internal vibrations of triiodomesitylene (TIM) is presented. It is known from X-rays diffraction at 293 K that the molecule has nearly D-3h symmetry because of the large delocalization of the methyl protons. By using Raman and infrared spectra recorded at room temperature, a first assignment is done by comparing TIM vibrations with those of 1,3,5-triiodo- and 1,3,5-trimethyl-benzene. This assignment is supported by DFT calculations by using the MPW1PW91 functional with the LanL2DZ(d,p) basis set and assuming C-3h symmetry. The agreement between the calculated and experimental frequencies is very good: always better than 97% for the observed skeletal vibrations. The calculations overestimate the methyl frequencies by 7%, and experiment shows only broad features for these excitations. Because a neutron diffraction study had established that the TIM conformation at 14 K is not exactly trigonal, new theoretical calculations were done with C, symmetry. This shows that all previous E ' and E" modes of vibration are split by 2-12 cm(-1). This is confirmed by infrared, Raman, and inelastic neutron scattering spectra recorded below 10 K. Apart from two frequencies, all the TIM skeleton vibrations have been detected and assigned by using C, symmetry. For the methyl vibrations, experiment has confirmed the splitting of the previously degenerate modes; only some small discrepancies remain in the assignment. This is partly due to the difference of the model conformation used in the calculations and the crystallographic one. All these results confirm that each of the three methyl groups has not only its own tunnel splitting but also a different specific spectroscopic behavior for all the molecular modes.