Previously compiled thermodynamic quantities for gaseous species in (gallium + chlorine) are revised. Entropies and heat capacities of Ga 2Cl 6, Ga 2Cl 2, and Ga 2Cl 4 are re-calculated by the harmonic-oscillator rigid-rotator method taking account of recent spectroscopic quantities. Enthalpies of formation of GaCl, Ga 2Cl 2, GaCl 2, and Ga 2Cl 4 are deduced from vapour pressures measured by Bourdon gauge over {Ga(l)+GaCl 3(g)} and {GaAs(s)+GaCl 3(g)}. For (gallium + chlorine), enthalpy determinations were performed on the assumption that the gaseous phase is represented firstly by the major species GaCl and GaCl 3, and secondly by GaCl, GaCl 3, GaCl 2, Ga 2Cl 2, and Ga 2Cl 4. Another calculation was made taking account of the dilution of chlorine in the condensed phases. For (gallium + arsenic + chlorine), thermodynamic calculations were performed with a simple gaseous phase: GaCl 3, GaCl, As 4, and As 2; and with a complex gaseous phase represented by all the (gallium + chlorine) gaseous species. The results are discussed and allow us to propose a new set of formation ethalpies: Δ f H m o(298.15 K)/(kJ·mol −1): GaCl, −(68.7 ± 5.1); Ga 2Cl 2, −(159±28); GaCl 2, −(130±29); Ga 2Cl 4, −(599±18).