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.