The surface structures of ZrO2, Y2O3-ZrO2 and SiO2-ZrO2 catalysts were studied in relation to their basicity and acidity, and the relationship between surface structure and gasification gas clean-up activity was evaluated. The physical properties of the catalysts were characterized by BET, XRD and Raman spectroscopy. The basicity and acidity of the catalysts were measured by CO2-TPD and NH3-TPD, respectively. The types of active surface sites were determined by in situ DRIFTS experiments for the adsorption and desorption of CO2. The order of activity of the catalysts in the decomposition of tar from synthetic gasification gas was ZrO2 > Y2O3-ZrO2 > SiO2-ZrO2. Acidity was observed to be an unfavorable characteristic for ZrO2-based gas clean-up catalysts, whereas basicity seems to be essential. The redox properties of Y2O3-ZrO2 enhance its overall oxidation activity and the lower performance of Y2O3-ZrO2 in tar decomposition compared to ZrO2 can be explained by its oxidation activity towards other gas compounds than tar. (C) 2009 Elsevier B.V. All rights reserved.