The hitherto unknown structures of stable crystal phases II and IV of undeuterated 1,3,5-trichloro-2,4,6-trimethylbenzene, TCM9H (phase II, above T III→II =314K, and phase IV, below T III→IV ≃160K, phase III in between) have been solved ab initio from powder synchrotron X-ray diffraction data, respectively, at 343K by direct methods and at 50K using a Monte-Carlo simulated annealing method. They have been then refined for perdeuterated TCM (TCM9D, same behaviour as TCM9H) against high‐resolution powder neutron diffraction data at 343 and 2K, respectively. Phase II is disordered (space group P2 1 /n, Z=2, a=15.12984(13)Å, b=3.92080(3)Å, c=8.27786(8)Å, β=90.8374(8)°), whilst phase IV is ordered (space group P1¯, Z=2, a=7.42872(5)Å, b=8.75731(6)Å, c=8.76246(6)Å, α=59.8543(4)°, β=68.2959(5)°, γ=73.1654(6)°). The mechanisms driving both reconstructive phase transitions at T III→II and T III → IV are described at a microscopic level; it is shown that they involve rotations of the molecules (out-of-plane and in-plane, respectively, with regard to the molecular plane of phase III) leading to huge atomic displacements up to 2.0 and 1.2Å, respectively. The molecular conformation in phase IV is compared to the one obtained from DFT quantum chemistry calculations for an isolated molecule in order to extract the characteristics of the inter-molecular interactions and the deuteron nuclear densities are extracted at 2K.