The liquid-to-particle mass transfer in micro packed beds of different hydraulic to particle diameter ratio N and different channel geometries, i.e. circular and rectangular, was studied with the copper dissolution method. In this paper, we demonstrate that the shape of the channel has no influence on the liquid-to-particle mass transfer as long as N is constant. Moreover, higher Sherwood numbers were measured with increasing N for 4 < N < 24.4, but the opposite influence was observed for N < 4. In the case of N > 4 we highlight that the channeling effect, i.e. higher velocities in the wall region compared to the center of the packed bed, has a main impact on the mass transfer. For N < 4 the packed bed cross section cannot be differentiated anymore in a wall region and a center region, hence there is no wall effect and a better flow distribution through the packed bed cross section is induced which increases mass transfer, as proposed in this paper. (C) 2011 Elsevier Ltd. All rights reserved.