The gas-phase hydrogenation of 3-methyl-butenal or prenal, was studied, at temperature equal to 353 K, on the (110) plane of platinum and compared to previous results obtained on Pt(111). The selectivity was found to be very structure sensitive. This has been ascribed to steric factors which determine when the unsaturated bond is on a substituted carbon atom. The combination of the use of well characterized surfaces as catalysts, and of the determination of the kinetics of the reaction led us to propose a Langmuir-Hinshelwood type mechanism. The nature of the reaction intermediates, di-sigma or tetra-sigma activated adsorbed species, depends upon the reactant pressure and upon the geometry of the surface. This catalytic study on model surfaces is of particular interest in understanding the selectivity of real catalysts whose atomic arrangement is possibly defined and controlled.