The reactivities of an alpha,beta-unsaturated aldehyde, 3-methylcrotonaldehyde, and of its two monohydrogenated products, 3-methyl-crotyl alcohol and 3-methyl-butyraldehyde, have been investigated on a well-defined Pt(111) surface by low-pressure adsorption, thermal desorption, and high-pressure gas-phase hydrogenation experiments. Two kinetic regimes have been found when varying the 3-methyl-crotonaldehyde partial pressure and, in both cases, a rate-determining step has been proposed. At the origin of the reaction the high selectivity for 3-methyl-crotyl alcohol can be accounted for by the nature of the most abundant C5H9O isomer adsorbed species, the latter being determined by geometric effects. The influence of the 3-methyl-crotonaldehyde partial pressure on selectivities can be easily explained by a competitive hydrogenation between this molecule and the 3-methyl-crotyl alcohol. A similar previous study on Pt(100) has shown a quite different behaviour, and this work underlines the importance of the crystalline orientation of the platinum surface on the observed selectivities.