The structures of the catalytically active sites in supported metal catalysts are a long sought after goal. In this study, XAS has been used to establish these structures. The ethene-induced changes in the XAS spectra as a function of temperature and pressure were correlated to changes in the adsorption mode of the hydrocarbon. At low temperature, ethene was adsorbed in on-top (pi) and bridged (di-sigma) sites on small platinum clusters. Below room temperature, the adsorbed ethene was dehydrogenated to an ethylidyne species, which was adsorbed in threefold Pt sites. On larger clusters the dehydrogenation proceeded at higher temperature indicating a different reactivity. EXAFS results showed that changes in the geometrical structures were mainly due to (co)adsorbed hydrogen. Our results for platinum agree with those obtained using other techniques proving that detailed shape analysis of the L-3 edge XANES is a practical tool to determine the structure of the sites that are involved in bonding to reactants and intermediates. Application to gold and alloy catalysts showed that ethene induced a significant change in the electronic structure of gold nanoclusters that could be interpreted as ethene adsorbed on top of single gold atoms or in bridged sites. Ethene adsorbed on both platinum and gold in the bimetallic clusters.