The usual empirical rule stating that the C = C bond is more reactive than the C = O group for catalytic hydrogenations of unsaturated aldehydes is invalidated from the present study. Density functional theory calculations of all the competitive hydrogenation routes of acrolein on Pt(1 11) reveals conversely that the attack at the C = O bond is systematically favored. The explanation of such catalytic behavior is the existence of metastable precursor states for the O-H bond formation showing that the attack at the oxygen atom follows a new preferential mechanism where the C = O moiety is not directly bonded with the Pt surface atoms, hence yielding an intermediate pathway between Langmuir-Hinshelwood and Rideal-Eley general types of mechanisms. When the whole catalytic cycle is considered, our results reconcile with experimental studies devoted to hydrogenation of acrolein on Pt, since the desorption step of the partially hydrogenated product (unsaturated alcohol versus saturated aldehyde) plays a key role for the selectivity.