Abstract The evaluation of the thermodynamic properties and phase diagrams of the FeO–SiO 2, NiO–SiO 2 and SiO 2–NiO–FeO systems is presented in which a structural model is used for the liquid phase. This thermodynamic model is based on the assumption that each metallic oxide produces the depolymerization reaction of silica network with a characteristic free energy change. A least squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously for the binary systems. In this manner, data for these binary systems have been analyzed and represented with a small number of parameters. The binary structural model is extended to the ternary system assuming a random mixing of cations Fe 2+ and Ni 2+, since the FeO–SiO 2 and NiO–SiO 2 binary systems exhibit similar thermodynamic behavior, that is comparable free energies of mixing.