Knudsen dynamics simulations were performed in cylindrical pores with multiple sections of different diameters. The number and location of hits between particles and the pore walls can be related to the known distributions in the uniform diameter cases. Both sweep-gas and pass-through modes of,operation were examined. The contact of the particles traveling inside the pores with the pore walls can be regulated by adjusting the aspect ratio (section length/section diameter) of the pore sections and the ratio of their cross-sectional areas. The results were found to be independent of the transition angle between sections except for very smooth transitions. Analytical expressions were developed to relate the transmission probability in pores of multiple sections to the transmission probabilities of the constituent sections. (C) 2007 Elsevier Ltd. All rights reserved.