The thermodynamic properties of ternary liquid Cu-Mg-Si alloys with a constant concentration ratio of xCu/xSi=7/3 were determined using a combination of different experimental methods. Enthalpies of mixing were measured by isoperibolic calorimetry, and magnesium vapor pressures were obtained by an isopiestic method. Partial thermodynamic properties of magnesium were derived from the vapor pressure data, and the composition dependence of the magnesium activities is given for 1173 K. Integral Gibbs energies of mixing for liquid alloys were calculated by a Gibbs-Duhem integration. The agreement of the thermodynamic data obtained from different methods was found to be very good.