The thermodynamic properties of the liquid and solid Ag-In-Sn alloys were determined, using an electromotive force (EMF) method with an eutectic mixture of (KCl + LiCl) as a liquid electrolyte. The measurements were performed over the temperature range from 635 K to 856 K along the cross-sections Ag0.5Sn0.5-In, Ag0.5In0.5-Sn and In0.5Sn0.5-Ag. The partial thermodynamic functions of indium and liquidus temperatures of the investigated alloys were found from the obtained EMF values. The integral thermodynamic properties of alloys at 750 K along the Ag0.5Sn0.5-In cross-section were calculated by Gibbs-Duhem integration. Experimental points E(T) of the Inx(Ag0.5Sn0.5)1-x isopleth; xIn=0.10 (+); 0.20 (□); 0.333(○); 0.40 (△); 0.50 (▽). Display omitted •The electromotive force method (EMF) with a liquid electrolyte was used to determine the activity of indium in liquid and solid alloys of the Ag-In-Sn system.•The partial thermodynamic functions of indium were calculated from the obtained EMF values.•The Gibbs-Duhem integration was used to calculate the integral thermodynamic properties of alloys at 750 K along the Ag0.5Sn0.5-In cross-section.