The sedimentation velocity of uncharged, nonaggregated silica spheres under gravity is strongly reduced after addition of small amounts of nonsedimenting small spheres. This reduction is largely due to surface irregularities on a nanoscale of the large spheres at which a limited number of small spheres adsorbs, leading to an increase of the hydrodynamic friction per particle. This adsorption also screens effectively any weak Van der Waals attraction between the large spheres, which despite its weakness, significantly influences the concentration dependence of settling in a pure solvent. The concentration dependence and magnitude of the large-sphere sedimentation velocity in a more concentrated dispersion of small spheres agree with the prediction by Batchelor that the small particles mainly manifest themselves as an effective viscosity increase.