The activities of zinc, a Zn , and magnesium, a Mg , from very low to high temperatures in the Mg–Zn binary system were evaluated for the first time from the relationship between the Gibbs energies of formation, δ f G¯ o T , of Mg 48 Zn 52 , Mg 2 Zn 3 , MgZn 2 and Mg 2 Zn 11 and their phase equilibria. The δ f G¯ o T values adopted were determined in our previous calorimetric studies. It was found that the a Zn and a Mg values in the compounds steeply changed from the minimum to maximum as a function of the composition in a narrow solubility range. Such a change was more emphasized toward low temperatures (3 K). Since one of the dominant factors for the composition change in a narrow solubility range in Mg 48 Zn 52 and Mg 2 Zn 3 is the formation of vacancies at the Zn site, the relative partial molar Gibbs energies of the vacancy formation, δ G Zn vacancy Zn , were estimated from the obtained a Zn values. At 298 K, the δ G Zn vacancy Zn values of Mg 48 Zn 52 and Mg 2 Zn 3 were 73.5 and 344.3 kJ mol −1 , consistent with about the same order of the enthalpy of the vacancy formation in Ni 3 Al (= 173.7 kJ mol −1 ) as determined by positron annihilation spectroscopy. When the symmetric atomic configuration at the stoichiometric composition was violated by the formation of vacancies, the change in relative partial molar value of lattice defects was found to be large.