The results of studying the Ag–Sb–Se system by measuring the electromotive force (EMF) of concentration cells with the Ag4RbI5 solid electrolyte with respect to silver electrode in the temperature region of 300–450 К are shown. The solid solutions based on the AgSbSe2 form along the section Ag2Se–Sb2Se3 and their homogeneity region is determined. Based on equations of EMF vs. temperature dependences, the partial molar functions of silver in certain phase domains of this system are calculated. The standard thermodynamic functions of formation and the standard entropies are determined for the solid phases Ag0.8Sb1.2Se2.2, Ag0.9Sb1.1Se2.1, and AgSbSe2 and also for the low-temperature modification Ag2Se. The values found for AgSbSe2 and Ag2Se are compared with the literature data.