The partial and integral enthalpies of mixing of molten, ternary Ag–Cu–Sn alloys were determined at 500, 700 and 900 °C using a Calvet-type microcalorimeter. Five sections in a compositional range from pure Sn to about 40 at.% Sn were investigated. The data were fitted using a Redlich–Kister–Muggiano polynomial, where the binary interaction parameters were taken from the literature. Additionally, the temperature dependence of the ternary interaction parameters was described analytically by a linear function. In the ternary Cu–Ni–Sn system, the enthalpies of mixing were determined at 1250 °C in five sections, starting from pure Sn to about 40 at.% Sn. Again, the ternary interaction parameters were fitted using the substitutional Redlich –Kister –Muggiano model. For both ternary systems and all mentioned temperatures, isoenthalpy curves were constructed for the integral molar enthalpy of mixing.