By means of dc-reactive sputtering, it is possible to vary the stoichiometry of deposited zirconium nitrides, by varying the molar fraction of N2 in an Ar–N2 gas mixture. In order to understand the origin of this effect, a theoretical model of reactive sputtering is devised. It is based on the study of reaction kinetics taking place at the surfaces of the cathode and the chamber walls. In fitting the model with experimental data, it turns out that one has to introduce the roles of Ar, N2, and N species. For reactive sputtering of ZrNz films, a good fit is obtained when it is assumed that the molar fraction of N is constant when the molar fraction of N2 increases up to about 75% (under our experimental conditions). Above this concentration of N2, the concentration of N has to increase. By the analysis of the theoretical model, general scaling laws between experimental parameters (current, pressure, and areas of the cathode and the chamber walls) are easily obtained.