The adsorption and reaction of trans-1,2-dichloroethene on Cu nanoclusters supported on a thin Al2O3 film grown on NiAl(110) have been studied as a function of temperature and Cu cluster size using molecular beams and temperature-programmed desorption. At 300 K, dechlorination is observed on clusters of all sizes; subsequent desorption yields benzene and acetylene as the main reaction products. The ratio of these is strongly dependent on cluster size; for small clusters only acetylene evolution is observed. With increasing cluster size the probability of trimerisation of acetylene to produce benzene increases, although the desorption of acetylene remains dominant. The adsorption and reaction are compared to previous work on a Cu(110) single crystal where benzene was the main desorption product after adsorption at 300 K. The cluster size-dependent yield of desorption products is attributed to the inability of small clusters to accommodate the dissociation products prior to the trimerisation reaction. (C) 2004 Elsevier Inc. All rights reserved.