AbstractSolid ThJ4dissociates thermally below 500°C to form ThJ4‐gas, molecular iodine vapour and, successively, the solid sub‐iodides ThJ3, ThJ2, ThJ and, eventually, the metal. By sublimating and dissociating solid ThJ4from a Knudsen cell, the partial pressures of ThJ4and J2are both determined simultaneously by means of a mass‐spectrometer and calibrated partly gravimetrically. The measurements reported here and complementary earlier DTA measurements allow to calculate the heats of formation and standard entropies of the various iodides as well as the phase diagrams of the system Th — J. The main results of the mass‐spectrometric effusion experiments are the stepwise dissociation of ThJ4, its unexpected high dissociation pressure as compared with published estimates and the probable existence of a stable lower iodide with compositio