The system TlGaSSe–GaS has been studied through physico-chemical analysis, differential-thermal (DTA), X-ray diffraction, microstructural analysis and microhardness and density measurements. It has been established that the system TlGaSSe–GaS is a pseudobinary part of ternary system Tl,Ga||S,Se. at room temperature, TlGaSSe based solid solution extends to 10 mol.% GaS, and the TlGaSSe solubility in GaS–12 mol.%.