GaAs–HCl–H 2 . Vapor phase etching of polycrystalline GaAs was applied to the determination of the dominating equilibria in the common GaAs chlorine process using a simple entrainment method (Knobloch et al. 1984). It was established that in the temperature range 550–810°C, the dominating gallium compound in the vapor is GaCl, even at higher HCl inlet concentrations. From the equilibrium constants of the reaction GaAs + HCl = GaCl + 1/4As4 + 1/2H2, the standard enthalpy and the standard entropy of this reaction at 680°C (950 K) have been determined: ΔH 0 950 = 148.8 ± 1.5 kJ·M−1 and ΔS 0 950 = 131.8 ± 1.3 J·(K·M)−1. According to the calculation, the standard enthalpy of this reaction at room temperature is equal ΔH 0 298 = 150.0 ± 4.0 kJ·M−1.