Gold catalysts supported on hydrotalcite (HT) with a Mg:Al molar ratio 4:2 were prepared by direct anionic exchange (DAE) or by deposition-precipitation (DP) with NaOH or urea, in order to investigate the influence of the synthesis parameters on the catalytic conversion in CO oxidation. The highest activity was obtained for Au/Mg4Al2 prepared by DAE. The influence of various parameters on CO conversion such as the HAuCl4 concentration in the initial solution, the washing procedure, the pre-treatment temperature, the precursor of HT and the gold loadings were investigated. This study confirms the poisoning effect of chloride. The average gold particle size is higher for the catalysts with higher residual chloride content. In order to remove the remaining chloride from the sample, different washing procedures were used, i.e. either with warm water or with ammonia. Changes in activity and particle size were observed according to the washing procedure. Higher calcination temperature increases the surface area of HT contributing to better dispersion of gold. A smaller particle size, as measured by XRD and TEM, is obtained on HT prepared from sulfate precursor rather than nitrate. (c) 2005 Elsevier B.V. All rights reserved.