Aldol condensation of acetaldehyde was investigated over silica supported magnesium and zirconium oxides. The acidic and basic properties of the catalysts were studied by TPD of NH3 and CO2 and IR spectroscopy of adsorbed pyridine and CO2. MgO/SiO2 catalyst was characterized by high content of both basic and acidic sites, while ZrO2/SiO2 contained mainly Lewis acid sites. All materials studied were shown to catalyze the aldol condensation of acetaldehyde with selectivity to crotonaldehyde of ca. 85%. The activity of the catalysts was found to be in the following order: ZrO2/SiO2 > MgO/SiO2 >> SiO2. To assess the role of acidic and basic sites in condensation reaction, pyridine and carbon dioxide were used as probe molecules for poisoning of the corresponding active sites during catalytic runs. The results pointed to the key role of Lewis acid sites in acetaldehyde condensation. A concerted mechanism involving Lewis and Bronsted acid sites of the catalysts is proposed on the basis of in situ IR spectroscopic studies. (C) 2010 Elsevier B.V. All rights reserved.