The crystallographic phase transition which 1,2,4,5-tetrachlorobenzene undergoes at about 188 K from a high-temperature monoclinic (P2₁/a) structure to a low-temperature triclinic (P1) structure has been studied by means of the ³⁵Cl nuclear quadrupole resonance transition frequencies measured as a function of temperature from 77 to 300 K. The transition is shown to be first order displacive, determining the order parameter and its temperature dependence, and deducing a critical exponent beta =0.309+or-0.012 which indicates that the system behaves as a 3D Ising-like model where the important forces are those of short range. Also, the measured spectra are shown to be consistent with the measured (P2₁/a) and proposed (P1) crystal structures, producing an assignment of the spectra to the various chlorine sites in both primitive unit cells. Additionally, a wide temperature range (approximately 165-205 K) where phases coexist in a stable form is found and analysed.