An extensive survey of phase equilibria and thermodynamic data (Delta H, S, C-p, Delta G) on solid and liquid phases is presented for Ga-Sb, S-Ga and S-Sb subsystems which includes tables of the limiting activity coefficients. The construction method for chemical potential diagrams has been applied in the form of log[a(Ga)/a(Sb)] versus log[P(S-2)/bar] plots, the usefulness of which is discussed. The ternary phase diagram has been estimated. Using minimization of Gibbs energy the equilibrium composition of coexisting phases in the S-Ga-Sb system has been evaluated regarding the determination of the maximum level of sulphur doping in GaSb single crystals grown by the Czochralski technique without encapsulant. The calculated concentration of dissolved sulphur in GaSb solid was 10(16)-10(17) atoms per cm(3), which is in good agreement with the experimentally measured values of about 10(17) atoms per cm(3). After exceeding about 1.5 x 10(18) atoms per cm(3) in the melt, the second phase (Ga2S) started to separate spontaneously.