The polymerization of acetylene was studied by thermal programmed reaction on nanoassembled model catalysts fabricated by soft landing of size-selected palladium Pd, (1 less than or equal to n less than or equal to 30) clusters on well-characterized MgO(100) thin films. In a single-pass heating cycle experiment, benzene, butadiene, and butene were catalyzed with different selectivities as a function of cluster size: palladium atoms selectively produce benzene, and the highest selectivity for butadiene is observed for Pd-6, whereas Pd-20 reveals the highest selectivity for butene. These results provide an atom-by-atom observation of the selectivity of small palladium model catalysts, and extend toward small cluster sizes the study of Ormerod and Lambert on real palladium catalysts, where a structure sensitivity for the cyclotrimerization was observed. (C) 2001 Academic Press.