The NO-CO reaction kinetics has been studied on Pd/MgO (1 0 0) model catalysts at low pressure by a pulsed molecular beam technique. The model catalysts have been prepared by epitaxial growth of metal clusters in UHV on cleaved MgO single crystals. Three samples with mean clusters sizes of 2.8, 6.9 and 15.6 nm have been studied. The reaction products are mainly N-2 and CO2. The steady-state reaction rate has been measured between 150 and 400 degreesC at low pressures (10(-9)-10(-6) Torr). The rate limiting step for the reaction is the dissociation of NO at low temperature and the adsorption of CO at high temperature. The reaction probability of NO has been accurately determined, taking into account the reverse spillover of NO from the MgO support towards the Pd particles. It is shown that in these conditions the turnover number is not an appropriate parameter to determine the intrinsic effect of particle size. The intrinsic activity depends not only on particle size but also on particle shape. The medium sized particles exhibiting mainly (1 1 1) facets are found to be more active. The largest particles, which exhibit principally (1 0 0) facets, are less active.