The polythermal solubility diagram of the system K2SO4-MgSO4-H2O presents the formation of three double salts, picromerite, leonite, langbeinite appearing in this order with increasing temperature. In the temperature range between 314.15 K and 320.65 K, picromerite and leonite ought to coexist. The search in the literature revealed a lack of isothermal phase equilibrium data within this temperature range. Therefore, the solubility in the system K2SO4-MgSO4-H2O was determined in the whole concentration range at 318 K. The solid phases, epsomite, leonite, picromerite and arcanite occur with increasing potassium sulfate concentration. A two-salt point of leonite and picromerite is established at 0.618 molal K2SO4 and 3.030 molal MgSO4 at the temperature of investigation. (C) 2010 Elsevier B.V. All rights reserved.