1,2,4,5-Tetrachlorobenzene (TeCB) exhibits a phase transition α[RIGHTWARDS ARROW]β at 185 K whereas 1,2,4,5-tetrabromobenzene (TeBB) exhibits a phase transition β[RIGHTWARDS ARROW]γ at 307 K, the `room temperature' form being β for the two compounds. This work establishes which are the isomorphous forms. Quenching the molten mixtures into liquid nitrogen produced molecular alloys (TeCB)1 −x(TeBB)x at 295 K with any value of x, even in the 0.20 ≤x≤ 0.75 and 0.81 ≤x≤ 0.97 ranges where these molecular alloys are only metastable because of demixing phenomena. Powder diffraction analysis on the whole range of molecular alloys proved that βTeCB is isomorphous in the strictest sense (i.e. capable of giving a continuous series of mixed crystals) with the γ form of TeBB.