We have presented a model for reactive sputter deposition of two-phase materials. This model has been applied to reactive sputtering of chromium nitride where it is assumed that either Cr2N or CrN is formed. In order to test the validity of the model, a number of deposition experiments has been performed where the film composition for different reactive gas supplies has been estimated by means of in situ soft x-ray emission spectroscopy. The results have been compared with predicted film compositions according to the model and the experimental findings agree qualitatively with the simulations.