Thermodynamics of the equilibrium phase assemblage Ag0.9Cu 1.1S-Cu2-xS-CuS have been studied by an EMF-method. The ternary phase was synthesized from the pure Cu2S and Ag2S in vacuum furnace at high-temperature and mixed with appropriate compositions of Cu2S and CuS to form the cathode electrode. The EMF-measurements were made on the solid-state galvanic cell Pt(-) | Ag | RbAg4I 5 | Ag0.9Cu1.1S-Cu2-xS-CuS | Pt(+), in a temperature range from 316 K to 498 K. Based on the measured EMF vs. T relations, thermodynamic functions (ΔfG°, Delta;fS° and Delta;fH°) of stromeyerite (Ag 0.9Cu1.1S) in equilibrium with Cu2-xS and CuS have been determined. The obtained experimental values have been compared with the available literature values. New experimentally determined thermodynamic properties of the solid solution (Ag,Cu)2Sss were determined.