Thermodynamic modelling of the iron-nickel sulphide phase violarite, nominally FeNi2S4, has been carried out by critical evaluation of experimental data on ternary phase equilibria at 0.1 MPa total pressure. A three-sublattice approach in the framework of the compound energy formalism is developed for a consistent integration of the violarite solid solution in a complete thermodynamic description of the iron-nickel-sulphur system. Selective iron occupation on octahedral sites within the spinel lattice of violarite reported in the literature is supported for the first time by thermodynamic modelling. (C) 2008 Elsevier Ltd. All rights reserved.