A significant number of models allowing the calculation of water activity (a(w)) in aqueous solutions of electrolytes or of organic substances was proposed and verified experimentally in the last few decades. In case of a mixture of organic and inorganic solutes the lowering of water activity with respect to corresponding single solutes was often observed but, to the best of our knowledge, did not find sound physical explanation yet. Present work proposes a thermodynamic model covering wide range of solution concentrations and applicable to multiple solutes, in particular to the mixtures of high total solid contents, situation relevant to powder caking. Deliquescence lowering is evidenced by means of qualitative visual observation and quantitative measurements of water uptake. Excellent agreement between the measured values of aw for different compositions of NaCl-sucrose-water mixtures and the model calculation of the water activity at any composition is obtained. A diagram relating water activity to the composition of a ternary system is proposed. It facilitates foreseeing the conditions of ambient relative humidity where partial or total dissolution of mixtures of NaCl and sucrose will take place. In addition, it furnishes a better understanding of deliquescence lowering reported in the earlier works. (C) 2012 Elsevier Ltd. All rights reserved.