The total energies and lattice constants of binary hcp-and fcc-TiN, AlN and ternary Ti 0.5 Al 0.5 N phases are calculated by ab initio method using the Vienna ab initio simulation package (VASP). The values of total energies are then used to calculate the lattice stabilities of binary hcp-and fcc-TiN, AlN and the interaction parameter of ternary Ti 1-x Al x N phases on the basis of the semiempirical, thermodynamic sub-lattice model. Based on these data, the Gibbs free energy diagram of the immiscible quasi-binary TiN-AlN system are constructed in order to discuss the relative phase stability of the metastable ternary hcp-and fcc-Ti 1-x Al x N phases over the entire range of compositions. The prediction is compared with the published results from PVD and CVD experiments. The calculated lattice energy and the constructed Gibbs free energy diagram show, in agreement with the experiments, that metastable fcc-Ti 1-x Al x N coatings can easily undergo spinodal decomposition into coherent fcc-TiN and fcc-AlN, but there is a relatively large barrier for the formation of the stable hcp-AlN. A comparison with the TiN-Si 3 N 4 system shows that, due to the much higher de-mixing energy of this system as compared to the TiN-AlN one, spinodal decomposition may occur in that system also for semicoherent TiN and Si 3 N 4 phases.