On the basis of a critical assessment, experimental investigations by EPMA on ternary Al-Mg-Zn alloys were specifically performed to provide missing data of the ternary solubilities of the AI-Mg and Mg-Zn phases as well as to improve the knowledge of the extensions of the homogeneity ranges of the ternary tau- and Phi-phases. A thermodynamic description for the Al-Mg-Zn system was obtained taking into account those experimental data together with constitutional, thermodynamic and crystallographic literature information. The binary intermetallic phases are modelled to have ternary solubilities. The ternary tau-phase is modelled according to its crystal structure with cubic symmetry as (Mg)(26)(Mg, Al)(6)(Al, Zn, Mg)(48)(Al)(1) in the compound-energy-formalism. The Phi-phase is described by the sublattice formula Mg-6(Al, Zn)(5).