Burkeite is a double sulphate and carbonate of sodium salt Na6SO4(2-y)CO3(1+y) presenting a large non-stoichiometry. Published models for solid-liquid equilibrium of burkeite are either of quite poor predictive quality or insufficiently based on thermodynamic grounds. We propose a model based on thermodynamics and tested against experimental data, where the precipitation of burkeite is depicted as the precipitation of a reference crystal in which anion substitution occurs. The reference crystal was determined as composed of sodium and sulphate only, where half the anion sites are available for substitution by carbonates. These results are consistent with earlier published crystal observations. (C) 2011 Elsevier Ltd. All rights reserved.