Ce and Mg were used as promoters in two series of Ru based catalysts supported on alumina (Al2O3) and activated carbon (AC). The catalysts were characterized by H-2 chemisorption and temperature-programmed reduction (TPR), and studied in the crotonaldehyde (gas phase) and the citral (liquid phase) hydrogenations. Addition of MgO and CeO2 decreased the catalytic activity in crotonaldehyde and citral hydrogenations. With regard to the selectivity towards unsaturated alcohols. similar trends were observed for the two reactions. MgO did not influence the selectivity, but CeO2 increased the selectivity to unsaturated alcohols, especially on carbon supported catalyst. Bulk CeO2 and Ce/AC catalyst showed low activity but very high selectivity (93 and 100%, respectively) to the unsaturated alcohols. Based on these results and the calorimetric experiments of CO adsorption it was suggested that defect sites on the surface of the promoter are the active and highly selective sites for unsaturated aldehydes due to their influence on the C=O bond activation.