The solubility and density in the thermodynamic phase equilibria ternary system (Li2SO4 + K2SO4 + H2O) at 288.15 K and 0.1 MPa were investigated experimentally with the method of isothermal dissolution equilibrium. This system at 288.15 K consists of two invariant points, three univariant isothermal dissolution curves; and three crystallization regions. The salt Li2SO4 center dot K2SO4 belongs to the incongruent double salt, and no solid solution was found. Based on the Pitzer model and its extended Harvie-Weare (HW) model, the mixing ion-interaction parameters of theta(Li,K,) psi(Li),(K,SO4) at 288.15 K and the solubility equilibrium constants K-sp of solid phases Li2SO4 center dot H2O and Li2SO4 center dot K2SO4, which are not reported in the literature were acquired. A comparison between the calculated and experimental results at 288.15 K for the ternary system shows that the calculated solubilities obtained with the extended HW model agree well with experimental data. (C) 2015 Elsevier B.V. All rights reserved.