The phase diagram of the GaSe-TlGaS2 system has been studied using differential thermal analysis, X-ray diffraction, microstructural analysis and microhardness and density measurements. It has been established that the system is a nonquasibinary part of quasiternary system GaS-GaSe-TlS. At room temperature the GaSe –based solid solution extends to 3,5 mol % TlGaS2 , and the TlGaS2 solubility in GaSe is 5 mol %.