A complete, critical evaluation of all phase diagrams and thermodynamic data was performed for all condensed phases of the (NaCl + Na2SO4 + Na2CO3 + KCl + K2SO4 + K2CO3) system, and optimized parameters for the thermodynamic solution models 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 order, where the cations (Na+ and K+) were assumed to mix on a cationic sublattice, while anions (CO32-, SO42-, and Cl-) were assumed to mix on an anionic sublattice. The thermodynamic properties of the solid solutions of (Na,K)(2)(SO4,CO3) were modelled using the Compound Energy Formalism, and (Na,K)CI was modelled using a substitutional model in previous studies. Phase transitions in the common-cation ternary systems (NaCl + Na2SO4 + Na2CO3) and (KCl + K2SO4 + K2CO3) were studied experimentally using d.s.c./t.g.a. The experimental results were used as input for evaluating the phase equilibrium in the common-cation ternary systems. The models can be used to predict the thermodynamic properties and phase equilibria in multicomponent heterogeneous systems. The experimental data from the literature are reproduced within experimental error limits. (C) 2007 Elsevier Ltd. All rights reserved.