The characterisation of nanocatalytic materials was performed by pulsed molecular beam studies. Dual piezo-controlled pulsed molecular beams were used to probe oxidation reactions through their ability to accurately control the mole fraction of reactants. By varying the size of mass-selected Pd clusters soft-landed onto thin MgO films, strong dependency of the cluster size in the catalytic oxidation of CO was observed. True size effects were clearly observed only when experimental results were compensated for in the incoming flux. This correction led to determination of the reaction probability, the most unambiguous method of characterisation. Strong parallels between the new results for small, mass-selected clusters and grown, mass-distributed clusters can be clearly seen. (c) 2008 Elsevier Inc. All rights reserved.