The water activities for the binary Li2SO4H2O and MgSO4H2O, as well as the ternary Li2SO4MgSO4H2O systems at 323.15 and 373.15 K were elaborately determined by isopiestic measurements. The relative deviation of the isopiestic molality of parallel samples in each experimental run is better than 0.2%. The experimental water activities in the binary systems were compared with literature data and good agreement was found. The results obtained show that at salt concentration below 3 m the isopiestic composition lines of the Li2SO4MgSO4H2O system obey the Zdanovskii rule, whereas above 3 m the composition lines positively deviate from the Zdanovskii rule.