Employing a multi-molecular-beam approach, we have measured the angular distribution of CO, molecules formed during CO oxidation under steady-state conditions on well oriented and shaped nm-sized Pd crystallites grown on an ordered alumina film. The experiment is combined with kinetic Monte Carlo simulations based on a realistic structural model. The results obtained allow us (i) to differentiate between local reaction rates on the particle nanofacets and (ii) to conclude that oxygen diffusion on and between the (111) facets is rapid compared to reaction. (C) 2002 Elsevier Science B.V. All rights reserved.