In this paper, we analyze the kinetics of irreversible adsorption of hard spheres from a suspension at rest onto a plane under the influence of diffusion and gravity. We have obtained analytical solutions valid in the low coverage limit of the adsorption kinetics. In order to investigate the adsorption kinetics up to higher coverages, we have also performed nonsequential Brownian dynamics computer simulations. It is shown that the widely employed dimensionless radius R-* (or, equivalently, the gravitational Peclet number Pe) cannot alone characterize the relative effect of diffusion and sedimentation in adsorption kinetics. The description of the adsorption process requires the introduction of an additional, independent dimensionless number, G(ad), which is a combination of the Peclet number and the bulk volume fraction. The adsorption kinetics is dominated by diffusion for G(ad)<1 and by sedimentation for G(ad)>1, irrespective of the value of R-*. In the case of R-*>1 and G(ad)>1 the observed kinetics is qualitatively similar to the predictions of the ballistic deposition model, although significant deviations are observed. When G(ad)>1, it is also shown that blocking effects due to the interaction with previously adsorbed particles are proportional to the volume fraction so that they can be unobservable until the adsorbing surface is nearly saturated. (C) 2003 American Institute of Physics.