The solubility isotherms for the ternary system Li2SO4-MgSO4-H2O and the quaternary system Li2SO4-K2SO4-MgSO4-H2O at T = 273.15 K were determined by the isothermal equilibrium method. A Pitzer-Simonson-Clegg (PSC) model was used to simulate the properties of the binary and ternary systems of the quaternary title system. The binary model parameters were fitted against selected reliable experimental data of water activity. The mixture model parameters were obtained by fitting them to the ternary solubility isotherms taken from the literature and determined in this work. The solubility isotherms of the quaternary title system were predicted by the PSC model and compared with the experimental results in this work. The excellent agreement between the predicted and experimental results indicates that the experimental results obtained in this work are reliable.