Supported metal clusters and nanoparticles are of fundamental and technical interest, their use in heterogeneous catalysis being of particular importance. Yet on the atomic scale, there is only limited knowledge about the dependence of adsorption behavior and catalytic activity on particle size and morphology. In order to contribute to an understanding of such relationships in the case of hydrocarbon reactivity, we have studied the interaction of ethene with alumina-supported Pd, Rh, and Ir particles. Aggregates ranging in size from a few atoms up to several thousand atoms were prepared by metal vapor deposition onto a planar oxide support under ultrahigh vacuum conditions. The adsorption and dehydrogenation of ethene have been investigated by infrared and photoelectron spectroscopy. The infrared data reveal the formation of both pi and di-sigma bonded species upon adsorption at 90 K. On Pd. this involves a net charge transfer from the adsorbate to the metal, as shown by infrared spectra of co-adsorbed carbon monoxide. Upon heating to room temperature, the ethene adlayers on Rh and Ir deposits dehydrogenate to ethylidyne. In the case of Pd particles, this process is accompanied by the desorption of a particle-size-dependent fraction of the ethene.