The solubility of lithium potassium sulfate double salt in water and in aqueous methanol has been determined at a temperature of 298.15 K, in the methanol mass concentration range 0-0.4 kg of methanol/kg of water. An equation is given for the solubility as a function of the composition of potassium sulfate in solution. The densities and viscosities of the saturated solutions are also reported. Equations are given for both properties as a function of the solubility.