Surface and catalytic properties of alumina-supported bimetallic Pd-Sn catalysts prepared via a colloidal oxide synthesis have been studied. In-depth characterization of the supported metallic particles has been performed by techniques such as XPS, FTIR(CO), or LEIS, indicating a strong surface enrichment by Sn and suggesting a modification of the electronic properties of Pd ensembles. EXAFS results also demonstrate the core shell structure of the supported particles in agreement with our previous study of the oxidation state of tin species by Sn-119 Mossbauer spectroscopy. This in-depth characterization of the bimetallic catalysts allows us to demonstrate the influence of both PdxSny alloy formation and particle aggregation state on the selectivity of buta-1,3-diene hydrogenation. (C) 2003 Elsevier Inc. All rights reserved.