The tetrahedron approximation of the cluster variation method (CVM) was applied to describe the ordering of N atoms on the fcc interstitial sublattice of γ-Fe[N] and γ′-Fe4N1−x . A Lennard-Jones potential was used to describe the dominantly strain-induced interactions, caused by the misfitting of the N atoms in the interstitial octahedral sites. The γ-Fe[N]/γ′-Fe4N1−x miscibility gap, short-range ordering (SRO), and long-range ordering (LRO) of nitrogen in γ-Fe[N] and γ′-Fe4N1−x , respectively, and lattice parameters of γ and γ′ were calculated. For the first time, N distribution parameters, as calculated by CVM, were compared directly to Mossbauer data for specific surroundings of Fe atoms.