A control system for stabilizing the reactive magnetron-sputter process is applied that is based on a combination of a short-term stabilization of the plasma and a long-term stabilization employing an in situ spectroscopic ellipsometer. For the formation of niobium pentoxide, a measurement of the reactive gas partial pressure with a lambda-probe measurement was employed. It turns out that the specific deposition rate, i.e,, deposition rate divided by the power density, is a suitable control parameter. Compared to the oxide model the deposition rate of niobium pentoxide films deposited in the transition mode could be enlarged by a factor of more than 4. The films were grown at different process parameters (oxygen partial pressure, target power, absolute pressure, midfrequency and de technique) onto unheated substrates. In situ! spectroscopic ellipsometry and ex situ spectroscopic ellipsometry at different angles of incidence and scanning electron microscopy investigations were applied to study the optical properties and the morphology of the films.