The density separation processes in the size range ;1 mm depend on differences in the drag velocities in a thin layer of water flowing down an inclined plane. The models of the drag velocities of particles as proposed by FINKEY and by BAGNOLD are mentioned in textbooks, but have never been applied for quantitative predictions of technical separation results. The main scope of the thesis was to check the applicability of the two said models by a series of experiments based on new methods of measuring the settling velocities and the drag velocities of mineral particles in polydispersed suspensions. Additional measurements concerned the viscosity/concentration-functions of the pulp samples. The comparison of the experimentally determined drag velocities versus calculated velocities gave clear evidence that both models provide a qualitatively correct information on the influence of particle size and particle density, but fail completely when quantitative information is sought. The conclusion is that at present there is no alternative to common separation tests when technical separation results are to be predicted. (Author, shortened by G.Q.).