Phase equilibria in the Tl-TlI-S composition region of the Tl-S-I system were studied by differential thermal analysis, x-ray powder diffraction, and measurements of the microhardness and the emf of concentration circuits relative to a thallium electrode. A series of polythermal sections, an isothermal section at 300 K, and a projection of the liquidus surface were constructed. Primary crystallization regions of six phases, including the ternary compounds Tl6SI4 and Tl3SI, were outlined, and the types and coordinates of non- and monovariant equilibria were determined. It was shown that the ternary compound Tl6SI4 forms tie lines with Tl, TlI, Tl2S, Tl4S3, and TlS in the subsolidus region and that the homogeneity region of Tl6SI4 below 400 K does not exceed 1 mol %. From the emf measurement data, the standard thermodynamic functions of formation and standard entropy of the compound Tl6SI4 were calculated: G ,298 0 = −601.7 ± 2.5 kJ/mol, ΔH ,298 0 = −595.1 ± 4.0 kJ/mol, and S 298 0 = 672 ± 10 J/(mol K).