Core-shell RuPt (Ru core-Pt shell) and PtRu (Pt core-Ru shell) nanoparticles were prepared by decomposing in a two-step procedure a ruthenium ([Ru(COD)(COT)] (COD = 1,5-cyclooctadiene, COT = 1,3,5-cyclooctatriene)) and a platinum complex ([Pt2(dba)3] (dba = dibenzylideneacetone) or [Pt(CH3)2(COD)]) in the presence of 4-(3-phenylpropyl)pyridine (PPP) as a stabilizer and using different Ru/Pt ratios. The data obtained from a combination of several analyses (TEM, HRTEM, WAXS and IR) indicate that the so-obtained nanoparticles present a core-shell structure. The catalytic performances of these bimetallic nanoparticles for the selective hydrogenation of trans-cinnamaldehyde were investigated, which provided interesting results as well as useful information to elucidate their structure and composition. Indeed, the catalytic results evidence that: (1) the combination of both metals led in some cases to a synergistic effect on the selectivity of the reaction, and (2) the structure and the composition affected the selectivity of the reaction.