A SiO2 suspension with a strictly defined particle size distribution and a sediment taken from the Vistula river were sepd. by short and long-term fractionations carried out in a newly developed self-adjustable split-flow thin cell fractionation system operating in the full depletion mode. The collected fractions were characterized by using a laser diffraction particle size analyzer. A very good sepn. of both materials during the short-term fractionation was achieved. The sepn. efficiency significantly decreased during the 24 h long fractionation because of sedimentation of particles in the app.