In fixed bed catalytic reactors radial heterogeneities of the granular structure are present owing to topologic constraints imposed by the reactor wall. In order to analyse the influence of this structure on the fluid flow and the radial mass transfer properties, the study of sphere packings in cylindrical container and flow simulations at the pore scale are carried out. A collocated finite volume is used to solve the 3D Navier-Stokes equations. The Reynolds number ranges from 7 to 200 allowing to use the direct numerical simulation method in stationary flow regime combined to the no-slip condition at the interface solid/fluid. Therefore no correlation are used in this study. Furthermore, representative fixed beds, composed of several hundreds of spheres, are used for a diameter ratio of 5.96 and 7.8. Radial profile of the longitudinal velocity and the probability density function of the velocity components agree with the experimental data found in the literature. At low Reynolds number, the computation of current lines reveals the presence, at the reactor wall, of a layer whose width is around one-fourth of the sphere diameter. In this layer, the fluid flow is longitudinal and tangential. Particle tracking reveals also the existence of a second layer all along the spheres in contact with the reactor wall. Mass transfer in these two regions is controlled by the diffusive mechanism at low Reynolds number. The flow structure at high Reynolds number contains lots of eddies distributed homogeneously in the fixed beds. These structures are not recirculating zones (particle traps). On contrary, they accelerate the radial mass transfer so that the layers found at low Reynolds number tend to disappear at high Reynolds. (C) 2003 Elsevier Ltd. All rights reserved.