The purpose of the present work is to provide a realistic diffusion-precipitation model, which describes the volume diffusion of nitrogen in ferrite and the simultaneous precipitation of fine scale alloying element nitrides in the diffusion zone. This model is based on the finite-volume method, coupled with a thermodynamic description of the considered ternary system. The thermodynamic data related to the formation of nitrides are included in the model, as the precipitation depends on local variables of the system like temperature and solute concentrations. To validate our calculations, Fe-0.55% V and Fe-1.62% Mn alloys were nitrided at 843 K. Theoretical results are in very good agreement with the experimental measurements. The model can predict the distribution of nitrogen in ferrite phase as well as nitrogen presents in the alloying element nitride precipitates, and excess nitrogen around the precipitates. The present model describes the influence of any variable that affects the nitriding depth, or the nitrogen concentration in the diffusion zone. This model can be easily generalized to iron containing several alloying elements.