The sedimentation in aqueous suspensions of surface-coated silica and α-alumina has been investigated by means of time domain dielectric measurements in the frequency region 30 MHz to 5 GHz. The particles have been coated with a non-ionic surfactant, nonylphenol hexaethylene oxide (C 9 PhE 6 ), and a non-ionic polymer, ethyl(hydroxyethyl)-cellulose (EHEC). By applying theoretical models for the dielectric response we were able to follow the build-up of the sedimentation volume of the solid particles and thus to follow the difference in stability of the suspensions due to the surface modification of the solid particles.