The structure sensitivity of the catalytic hydrogenation of cinnamaldehyde was studied by carrying out comparative studies with Pt/SiO₂ catalysts of various metal loadings, between 0.5 and 5.0 Pt wt%. Transmission electron microscopy (TEM) characterization was complemented with carbon monoxide adsorption experiments using infrared absorption spectroscopy and with kinetic studies under high (10 bar) pressures of H₂. Specific total turnover frequencies, normalized by the number of exposed Pt atoms, were found to correlate with both the average diameter of the metal nanoparticles and the average number of CO molecules adsorbed on flat terraces. It was concluded that cinnamaldehyde conversion is likely to take place on close-packed (111) planes. The selectivity between the hydrogenation of CC versus CO bonds, on the other hand, appears to be independent of either the particle size or the extent of the conversion, at least within the Pt particle size range explored in this study.