ZnO-CO/SiO2 catalysts with various cobalt loadings and Co/Zn ratios were studied in terms of both their surface structure and catalytic hydrogenation of crotonaldehyde (CROALD), to investigate the influence of zinc on the properties Of CO/SiO2 catalysts. The ZnO-CoOx precursors were prepared in situ on the silica support by co-impregnation of the support with an aqueous solution of cobalt and zinc nitrates. The oxide precursors and catalysts were characterized by temperature-programmed reduction (TPR), temperature-programmed desorption of hydrogen (TPD-H-2), and diffuse reflectance FTIR spectroscopy (DRIFTS) of adsorbed CO. The TPD-H-2 and DRIFTS of CO indicated that the presence of Zn suppresses formation of the type alpha sites present in catalysts containing only Co and promotes surface-structure formation of ZnOx-Co sites obtained by reduction of the catalysts at high temperatures. The presence of ZnOx in the surface structure blocks both CO adsorption in bridge form and the formation of the hydrocarbonyl structure in the presence of H-2. For crotonaldehyde hydrogenation in the gas phase, selective hydrogenation of C=C or C=O groups was correlated with the relative proportion of species in the oxide precursors. The formation of ZnOx-Co at the surface strongly suppresses the C=C hydrogenation activity. However, this surface structure is not stable during prolonged catalytic runs. After a time on stream, the catalytic properties revert to those of the unpromoted catalysts. (C) 2003 Elsevier B.V. All rights reserved.