Effect of catalyst pre-treatment conditions on activity and selectivity of sulfided Ni-Mo/gamma-Al2O3 catalysts in aldehyde hydrogenation reactions was studied. It appears that both of the sulfidation temperature and the post-sulfidation degassing temperatures can be effectively used to manipulate the surface density of anion vacancy sites and the hydroxyl and sulfhydryl sites over the sulfided Ni-Mo/gamma-Al2O3 catalysts. Any changes in the surface density of these sites, in turn, can have significant effect on the activity and product distribution achieved over these catalysts in hydrogenation of linear aldehydes, hexanal and propanal. As established previously, NO and CO2 can be used to probe the sites that promote the alcohol and heavy product formation reactions, respectively. The results obtained from XRD, XPS, DRIFTS, TPD and volumetric chernisorption measurements provide complementary information in establishing the correlations between the active sites and reaction performance. (c) 2005 Elsevier B.V. All rights reserved.