we study by means of direct numerical simulation the mixing performance of fixed and fluidized beds of spherical, monodisperse particles in narrow channels. Mixing performance is being characterized by an effective diffusion coefficient. The Reynolds numbers based on the channel width and superficial velocity are of the order of 10. Under these laminar conditions the mixing of a passive scalar is greatly enhanced by the presence of particles, with mobile particles (in fluidized beds) performing much better than fixed particles. The simulations allow for the design of micro-scale mixing devices in terms of reactor size (length and width), particle size, solids volume fraction, and flow rate. (C) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.