We present a detailed analysis of the thermodynamics of the BGaAs alloy formation taking into account the question of boron solubility and Gibbs free energy change. We also predict a Gibbs free energy change Delta(r)G(BAs) = -480 U/mol for the gas phase formation of BAs to be compared to Delta(r)G(GaAs) = -284kJ/mol for GaAs. We show that the experimental boron solubility is actually an open question both theoretically and experimentally, while the maximum concentration obtained up to date is reaching 6-7%. It thus follows that despite the low boron solubility, the Gibbs free energy change for the alloy formation is dominated by the chemical term over the mixing energy change. We could deduce order of magnitude for the ratio of the boron partial pressure and that of gallium. P-B could be estimated as being always much lower than P-Ga; it is shown that the alloy content is probably controlled by the Ga equilibrium partial pressure. (c) 2006 Elsevier B.V. All rights reserved.