Hydrogenation of aldehydes over sulfided Ni-Mo/Al2O3 catalysts was studied using hexanal and propanal as model feed compounds. NO adsorption and CO2 adsorption measurements, which are used to probe the coordinatively unsaturated sites (CUS) and the hydroxyl groups on the alumina surface of sulfided Mo catalysts, were found to correlate well with alcohol and heavy product formation rates, respectively. Static chemisorption measurements as well as diffuse reflectance Fourier transform infrared spectroscopy analysis were used in the quantification of the CUS and OH sites on the surface. The anion vacancies associated with Ni sites were found to have a higher intrinsic activity for hydrogenation of aldehyde to alcohol. Although CO2 can provide some measure of the exposed alumina surface, it tends to overestimate the surface coverage since it adsorbs preferentially on more basic OH sites. Therefore, quantification of the OH groups relative to the bare alumina surface appears to provide a more accurate measure of the surface coverage, and to correlate better with the heavy product formation in aldehyde hydrogenation. (C) 2004 Elsevier Inc. All rights reserved.