According to the Mixing-Roughness-Information depth (MRI) model, the depth resolution function in sputter depth profiling can be described by three parameters: atomic mixing, roughness and information depth, which are assumed to be constant for all interfaces. The roughness parameter is composed of the original roughness of an interface and the roughness induced by ion sputtering. In principle, these two contributions can be separated. AES depth profiling of a SiO2/Ta2O5 multilayer sample was performed with 1 keV Ar* and ionized SF6, the original interface roughness was determined by means of grazing incidence X-ray reflectivity (GIXR) and is larger for the Ta2O5/SiO2 than for the SiO2/Ta2O5 interfaces, in qualitative accordance to the interface width measured from the depth profiles. After interruption of the sputter profiling at selected interfaces, the surface roughness after sputtering was obtained by AFM measurements. Comparison of these results with that of the MRI roughness shows that they are insignificant in comparison with the roughness parameter introduced by the blurring of the mixing length.