Hydrogenation of hexanal to hexanol was studied over sulfided Ni-Mo catalysts supported on gamma-Al2O3. Characterizations of the oxide and sulfide forms of the catalysts were performed using XRD, XPS, TPD, DRIFTS, and TEM/EDX techniques. The relationship between the molybdena surface coverage and the density of surface hydroxyl groups and their correlation with CO2 adsorption sites have been investigated. A strong correlation was observed between the selectivity of the catalysts towards heavy products, such as dimers, trimers and acetals and CO2 adsorption sites. The water resistance of the catalyst was also examined by keeping the catalyst on stream for 300 h. Post-reaction characterization of the catalysts performed using XPS technique showed partial oxidation of Mo sites and preferential loss of Ni from the surface. (C) 2004 Elsevier B.V. All rights reserved.