The thermodynamic and phase diagram data of the Li₂O-Al₂O₃ and Li₂O-MgO-Al₂O₃ systems were critically evaluated and optimized to obtain a set of consistent Gibbs energy functions for all phases in the systems. The LiAl₅O₈-MgAl₂O₄ spinel solid solution was modeled with the two-sublattice compound energy formalism by considering the cation distribution between the tetrahedral and octahedral sites and excess vacancy in octahedral sites. The liquid phase and monoxide solid solution were also described using the modified quasichemical model with pair approximation and the Bragg–Williams random mixing model, respectively. The thermodynamic models with optimized model parameters enable the reproduction of all reliable thermodynamic and phase diagram as well as spinel structural data in the system.