The reaction mechanisms between Sn-(n-C4H9)(4) and several specific types of surface sites on alumina are studied. A surface treatment is described allowing the complete elimination of reactive surface sites and avoiding the anchoring of Sn-(n-C4H9)(4) on the support. This surface deactivation is applied to the preparation of bimetallic PtSn/Al2O3 catalysts by controlled selective decomposition of Sn-(n-C4H9)(4) on the reduced metallic particles of a Pt/Al2O3 catalyst. The resulting bimetallic particles are analysed by Sn-119 Mossbauer spectroscopy at different stages of the preparation procedure. The tin anchoring sites on the platinum particles are identified on the basis of their specific Mossbauer hyperfine parameters. Tin atoms in contact with the support or with platinum are distinguished from those enclosed in bulk-SnO2-like agglomerates. A structural model is proposed describing at an atomic scale the morphology of the bimetallic particles and their evolution under various heat treatments. The results are compared with those of a catalyst prepared by conventional impregnation methods on a not specifically pretreated support. (c) 2006 Elsevier B.V. All rights reserved.