The thermodynamic properties of calcium–magnesium alloys were determined by electromotive force (emf) measurements using a Ca(in Bi)|CaF2|Ca(in Mg) cell over the temperature range 713–1048K. The activity and partial molar Gibbs free energy of calcium in magnesium were calculated for nine Ca–Mg alloys, calcium mole fractions varying from x Ca =0.01 to 0.80. Thermodynamic properties of magnesium in calcium and the molar Gibbs free energy of mixing were estimated using the Gibbs–Duhem relationship. In the all-liquid region at 1010K, the activity of calcium in magnesium was found to range between 8.8×10−4 and 0.94 versus pure calcium. The molecular interaction volume model (MIVM) was used to model the activity coefficient of Ca and Mg in Ca–Mg liquid alloys. Based on this work, Ca–Mg alloys show promise as the negative electrode of a liquid metal battery in which calcium is the itinerant species: alloying with Mg results in both a decrease in operating temperature and suppression of Ca metal solubility in the molten salt electrolyte.