The goal of the present work is to provide a fundamental approach to the thermodynamics of iron-based interstitial systems. A combination of ab initio total energy calculations, using the full-potential (linearized) augmented plane-wave plus the local orbital method including the magnetic contributions, and statistical thermodynamics is applied to model FCC-based Fe-N alloys. The ab initio calculated total energy of ordered compounds is used to obtain a set of volume-dependent effective cluster interactions (ECI’s) which parametrize the total energy of FCC-based Fe-N alloys. The g-Fe[N]/g’-Fe4N1-x phase boundaries and the distribution of N atoms over the octahedral interstitial sites are calculated by applying the tetrahedron approximation of the cluster variation method. The vibrational contributions are also considered by using the Debye-Grüneisen model. The current calculations of the phase boundaries and the N distributions agree well with data available in the literature. The interstitial N atoms show short range order in g-Fe[N] and long rang order in g’-Fe4N1-x.