The 35 Cl NQR spectra of 2,4,4,6-tetrachloro-3,5-dimethyl-cyclohexadiene-2,5-one-l and 2,4,4,6- tetrachloro-cyclohexadiene-2,5-one-l were studied from 77 K up to the decomposition temperature, their crystal structure having been determined by single crystal X-ray diffraction. The first compound crystallizes in the monoclinic space group C 2 2h -P2 1 /m with two molecules in the unit cell. The lattice constants are a = 887.2 (3) pm, b = 704.8 (3) pm, c = 824.0 (3) pm, β = 96.10(1)°. The second compound crystallizes orthorhombic, space group D 18 2h - Cmca, with 8 molecules in the unit cell which has the dimensions a = 711.2(2) pm, b = 1388.4(3) pm, c = 1729.8(4) pm. The structures determined belong to the stable phases of the title compounds as seen from the NQR spectra as a function of temperature which show no sign for a phase transition between the melting point and 77 K. In both title compounds the molecules are planar. Their symmetry is compared with other chloro- cyclohexadiene-2,5-ones-1 and chloro-cyclohexadiene-2,5-diones-1.4. The intramolecular distances d (C-Cl) of the sp 2 carbon atoms and the 35 Cl NQR frequencies assigned to the corresponding Cl atoms correlate quite well according to the theory: ν( 35 Cl) ~ (d (c-cl) ) -3 The relation is ν( 35 Cl)/MHz = 2 + 174 · 10 6 -(d/pm) -3 .