In many applications, reactive sputtering of compound films takes place in the presence of a bias voltage at the anode. In order to discuss fundamental processes involved in reactive sputtering, a model is given in which bias sputtering is introduced. It is assumed that no reaction takes place in the gas phase. Nevertheless, it turns out that the theoretical system involves 15 parameters that depend on the reactor design, the pumping system, the gas supply, the processing conditions, and the plasma-surface interactions. General equations taking all these parameters into account are given. Starting from these equations, many calculations can be performed. In order to show several possibilities for the model, we discuss the behavior of hysteresis in the pressure versus reactive gas flow curves.