The ChemSage code [Eriksson and Hack, Metall. Trans. B 12 (1990) 1013] to minimize the total Gibbs free energy was used to calculate phase equilibria in the complex thermochemical system representing LEC GaAs crystal growth which comprises the growth atmosphere, the liquid boron oxide, the GaAs melt and solid phases including the GaAs crystal. The behaviour of C, B, O, N and H in the crystal growth melt at 1509.42 K is investigated in dependence on relevant technological parameters.