The reciprocal system 3Tl(2)S + Bi2Se3 <-> 3Tl(2)Se + Bi2S3 has been investigated by DTA, X-ray powder diffraction analysis, and emf measurements. Some polythermal sections and the isothermal section at 500 K of the phase diagram and the projection of the liquidus surface of this system have been constructed, and the types and coordinates of the invariant and univariant equilibria have been determined. The existence of wide regions of quaternary solid solutions based on the binary compounds Tl2S, Tl2Se, Bi2S3, and Bi2Se3, and solid solutions between the temary compounds TlBiS2 and TlBiSe2 have been established.