An X-ray reflectivity study carried out on 45-450 angstrom films of radio frequency sputtered silicon oxide on silicon, with particular attention given to the interface between film and substrate. In order to model refectivity data it was necessary to include an interface layer for all films. This interface layer had a density approaching that of the substrate but due to differing compositions of the deposited film and substrate it was subject to a variation in scattering and absorption properties. (c) 2005 Elsevier B.V All rights reserved.