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The Fe–Ni system is a key subsystem for several alloys with important applications. It has been thermodynamically assessed according to the CALPHAD methodology and using 0 K enthalpies of formation of the ordered phases resulting from ab-initio atomistic calculations. This allowed us to model both stable and metastable fcc-based ordered phases (L1₂ Fe₃Ni, L1₀ FeNi and L1₂ FeNi₃) in the framework of the compound energy formalism (CEF) by using a 4-sublattice model. The combined ab-initio and CALPHAD approach enabled us to predict low-temperature stable equilibria which are experimentally not accessible due to an extremely sluggish kinetics. A similar 4-sublattice model has also been used for the bcc-based ordered phases (D0₃ Fe₃Ni, B2 and B32 FeNi, D0₃ FeNi₃), which are metastable in the Fe–Ni system, but need to be reliably modelled in order to enable extrapolations to higher order systems such as Al–Fe–Ni. Magnetic ordering, which is particularly important in this system, has also been thermodynamically described and the influence of magnetism on phase equilibria evidenced.