Control of the reactive sputtering process can be carried out by controlling either the flow of the reactive gas or the power supplied to the target. In the flow control mode it is well known that the reactive sputter deposition process exhibits a pronounced hysteresis effect. In this article we show that such a behavior is also observed in the power supply control mode. It is normally not possible to obtain stable operation of the system inside the hysteresis region. To achieve this, a feedback control system has to be used. In this way films with arbitrary compositions can be fabricated. Intermediate compositions, however, can also be obtained by varying the power fed to the sputtering target. Variation in film composition can be obtained by repeatedly switching the target power so that the process switches between the metal-and compound-sputtering modes. With this approach, it is possible to obtain any arbitrary stoichiometry of the film by choosing the correct ratio between the time in the metal mode and the time in the compound mode. In this article we present the physical background as well as experimental findings concerning operation in the power control mode.