Modeling and simulation at different scales were used to study mass transport and residence times of particles in nanostructured membranes with uniform cylindrical pores of 10-150 nm diameter and up to 5 mu m long. Analytical equations of the possible mass-transport mechanisms inside the pores were used to determine that diffusion dominates over convection under the conditions of interest for selective oxidation: 700K and pressure near atmospheric. Molecular dynamics simulations showed that surface diffusion is present only at temperatures < 700 K. Knudsen diffusion was identified as the dominant mechanism. Simulations based on its principles were performed using an ensemble of particles in a boundary-driven simulation cell, providing the average number of hits between a particle and the pore wall and the dependency of the residence time on the pore dimensions. The differences between operating a nanostructured membrane reactor in sweep-gas and pass-through modes were also investigated. (c) 2006 American Institute of Chemical Engineers AIChE J, 52: 3679-3687, 2006.