Thermodynamics of dissociation of metal iodides into metal and iodine atoms in metal halide arc discharges was investigated for ScI3/NaI/Hg, DyI3/Hg, CsI/Hg, and TlI/NaI/Hg systems at the temperature range of 1000–3000 K. Densities of the metal vapors calculated in the systems with the common thermodynamic quantities of metal atoms and iodides in vapor phases were compared with those experimentally determined from spectroscopic measurements or model calculations. For CsI and TlI, the experimental and calculated density values were closed to each other. On the other hand, the experimental values were much smaller than the calculated ones for sodium and rare-earth iodides because of diffusion of the iodides by chemical transport and convection in the discharges at high temperatures. Apparent Gibb’s energy changes of the dissociation in the lamps were expressed as first-order equations of temperature containing the terms of the increments of the average enthalpy and entropy changes, which were dependent on the molecular weight of the iodides. The densities of metal vapors derived with the obtained equations were comparable to those experimentally determined for all the systems under investigation. A series of equations were applicable to estimating the vapor density distribution of the other metal iodides, whose partial pressure distribution had never been experimentally estimated. © 1999 American Institute of Physics.