The TlAs2Se4-Tl3As2Se3Te3 system was studied using differential thermal analysis (DTA), powder X-ray diffraction, microstructure observation, and microhardness and density measurements. A phase diagram of the title system was constructed. This system is a quasi-binary join of the TlSe-As2Se3-As2Te3 quasi-ternary system. All alloys of the system under standard conditions are prepared in the glassy form. The system has a eutectic, which contains 50 mol % Tl3As2Se3Te3 and melts at 150A degrees C. The TlAs2Se4-base solid solution in the system extends to 12 mol % Tl3As2Se3Te3, and Tl3As2Se3Te3-based solid solution extends to 20 mol % TlAs2Se4.