We present the preparation, characterization and catalytic evaluation of gold-based catalysts over Al2O3, ZrO2 and 10% ZrO2/Al2O3 in selective CO oxidation. Oxidation and reduction pretreatments were evaluated using in situ X-ray diffraction, and the size of gold particles was determined. All catalysts contained gold particles in the range of 2-5 nm, with as-prepared samples containing a mixture of gold species (Au3+ and Auo). The catalytic tests showed complete CO oxidation at temperatures lower than 100 degrees C, with Au/ZrO2 sample presenting the best performance. Both the support and pretreatment conditions were found to cause changes to gold species. In situ X-ray diffraction patterns indicated the presence of different gold phases and of nanometer-scale crystallites of gold, which was confirmed by transmission electron microscopy results. It was also found that cationic gold improved the activity of Au/ZrO2. Concerning stability, time on stream tests showed that all samples were stable to coke deposition and sintering. (c) 2008 Published by Elsevier B.V.