A critical evaluation of all phase diagram and thermodynamic data were performed for the solid and liquid phases of the (Na2CO3 + Na2SO4 + Na2S + K2CO3 + K2SO4 + K2S) system and optimized model parameters were obtained. The Modified Quasichemical Model in the Quadruplet Approximation was used for modelling the liquid phase. The model evaluates first- and second-nearest-neighbour short-range ordering, where the cations (Na+ and K+) are assumed to mix on a cationic sublattice, while anions are (CO32- ,SO42-, and S2-) are assumed to mix on an anionic sublattice. The Compound Energy Formalism was used for modelling the solid solutions of (Na, K)(2)(CO3, SO4, S). The models can be used to predict the thermodynamic properties and phase equilibria in multicomponent heterogeneous systems. The experimental data from the literature were reproduced within experimental error limits. (c) 2006 Elsevier Ltd. All rights reserved.