The separation of particles with radii smaller than 1 mu m by means of migration induced by concentration gradients is considered. An experimental set-up consisting mainly of a measuring tube was used for the experimental investigations. The separating capacity could be enhanced from virtually zero in the absence of a concentration gradient up to 60% with a concentration gradient for particles with diameters of 0.135 mu m. The separating capacity was enhanced for larger particles from 25% to nearly 100% due to diffusion forces. The particle path in the measuring tube was calculated in order to compare these results with known results. Concentrations and velocities in the tube were calculated by means of a finite element program. The theoretical and experimental results are in good agreement.