Phase equilibria in the TlAs2Se3Te–TlAs2Se4 system has been studied using the methods of differential-thermal analysis, X-ray diffraction, microstructural analysis and microhardness and density measurements and state diagram has been plotted. It is established that the latter is a quasibinary section of the TlTe–TlSe–As2Se3 quaternary system. The solubility range of TlAs2Se3Te – based solid solutions is extended to 10 mol. %, and the region of TlAs2Se4 based solid solutions is extended to 7 mol. %. Under general conditions all the alloys of the system are received in a glassy form.