Thermodynamic properties of the interstitial solutions and phases in the Fe-C-N-O system are described by using the sublattice model. The temperature-depending parameters of systems (Fe-C, Fe-N, Fe-O) as well as changing in the Gibbs energy upon phase transformations of the interstitial elements in the solid and liquid iron are given. The coordinates of two-phase equilibrium lines are determined on the basis of computations of the phase diagrams and phase diagrams of Fe-C, Fe-N and Fe-O systems are plotted.