The heat contents and entropies have been derived as a function of temperature for gallium arsenide and the gaseous species which exist in equilibrium with it, in the temperature range 800°–1200°K when iodine or hydrogen chloride and hydrogen are also present. The important gaseous species are shown to be , , , , , I2, I, , As4, and H2. Under certain conditions the species As2, , and Ga (liq.) may also be present. The free energy changes for reactions among the above species are derived. They are shown to be consistent with those isothermal equilibria that have been directly measured. The free energy changes are used, together with the conservation equations for nondepositing components (I, Cl, H, and As in excess of the stoichiometric ) to give the partial pressures of all the important species at any temperature in a temperature gradient in a constant pressure system. The results are applicable to the calculation of the flux of gallium arsenide in open flow or closed tube (diffusive transport) systems, and they are in good agreement with experimentally measured rates of transport in the system in an open flow apparatus.