Although the Ag–Au–Pd system is crucial for several industrial applications and for the research on fundamental physics, no thermodynamic data on this ternary system at low temperatures have been reported in the literature. In the present study, activities of silver are directly measured by employing a solid-state EMF method, by using AgI as the solid electrolyte. The EMF was determined using a galvanic cell (Formula presented.), which produced novel experimental data on the thermodynamic properties of Ag–Au–Pd alloys. Darken method was used to calculate integral excess thermodynamic properties from the data. New thermodynamic characteristics, such as integral excess mixing Gibbs energy, entropy and enthalpy of the Ag–Au–Pd alloys, have been generated in a temperature range of 475 and 675 K. Isoactivity lines of silver in the system have been drawn throughout the Gibbs triangle. Thermodynamic properties of the binary Au–Pd alloys have been compared with the previous investigations.