A model of reactive sputtering, which includes plasma chemistry for the first time in d.c. magnetron discharges, has been developed. Argon and oxygen have been chosen as buffer and reactive gases, respectively, and silicon as the target material. We have developed a space averaged model of the plasma chemistry in a d.c. magnetron reactive sputtering process. We have calculated the rate coefficients of plasma chemical reactions and solved the plasma chemistry equations together with sputtering deposition equations. We have assumed different sticking probabilities of atomic oxygen created in plasma as compared with molecular oxygen. The model allows the investigation of the sputtering process and the plasma chemistry of the discharge for an arbitrary ratio of partial pressures of reactive and buffer gases. This makes it possible to simulate reactive sputtering for large erosion rates of target material. It is obtained that the dissociation of molecular oxygen in the discharge is so significant that the atomic oxygen is the main oxidizer for the target and substrate for almost all current densities. Numerical calculations also show that sputtering by oxygen ions can be quite significant, causing up to 1/4 of the total target erosion rate. The model describes well-known experimental results, for example, the hysteresis for small pumping speeds. Generally, deposition of any oxide film can be studied with this model.