A radiofrequency Ar/O-2 magnetron discharge is used to deposit silicon suboxide (SiOx, 0 <= x <= 2) films. In this paper we demonstrate how we can apply the high-energy ion beam technique elastic recoil detection (ERD) on-line, i.e. in situ during deposition, to continuously monitor the growing layer thickness and depth resolved composition. From the ERD data the deposition rates of distinctly silicon and oxygen and the composition of the growing film are determined. Using this method several growth conditions can be investigated and compared in a fast and reliable manner on a single substrate. In this work we report the variation of the growth rate and of the composition of the growing layer as a result of the variation of the rf power and show the consequence of keeping the O-2 flow constant in comparison to keeping the O-2 partial pressure constant. (c) 2005 Elsevier B.V. All rights reserved.