Rhodium-tin bimetallic particles entrapped in NaY cages were used to study the mechanism of tin-promotion in the selective hydrogenation of alpha,beta-unsaturated aldehydes. These model materials were obtained by chemical vapour deposition (CVD) and subsequent H-2 reduction of Sn(R)(4) (R=C2H5; C6H5) Onto reduced Rh/NaY samples that were prepared by ion-exchange (IE) or by chemical vapour deposition (CVD). In the former case, we have catalysts containing appreciable amounts of proton, while non-acidic metal-in-zeolite samples are obtained with CVD. TPRD studies indicate that the decomposition of tin precursors takes place on the surface of the rhodium particles only if the monolayer capacity is not exceeded. In addition, the mechanism of decomposition is influenced by protons and by the tin precursor used. Carbonyl DRIFT spectra reveal clear evidences of a surface tin-enriched Rh-Sn phase only for proton-free CVD-based samples. In this respect, Sn(C6H5)(4) leads to the formation of a higher tin coverage than that obtained from Sn(C2H5)(4). In the selective hydrogenation of citral (3,7-dimethyl-2,6-octadienal), the presence of protons was highly detrimental leading to the acetal formed by reaction with the solvent (ethanol). With proton-free catalysts, the formation of the saturated aldehyde and of the two unsaturated alcohols is observed. Selectivities could be influenced by both monolayer and multilayer deposits of tin on Rh/NaY, The promotion effect under running catalytic conditions is ascribed to the presence of non-ionic oxidised SnOx phases.