A preparation method for gold catalysts supported on alumina by a simple contact between a gold solution prepared with HAuCl4 as precursor and alumina over a narrow range of pH is described. This method, direct anionic exchange (DAE), differs from the mainly used method deposition precipitation generally used and presents the advantage that no gold is lost during the filtration and washing steps, the expected loadings of 2% is obtained. Gold particles size obtained by DAE is very sensitive to evolution and nature of the gold chloro-hydroxy species in the solution and on the surface of the support. According to the evolution of these species obtained by substitution of Cl- by OH-with increasing the pH value, the type of bonding complex-support is proposed. The surface of alumina is also sensitive to changes in pH of the solution. The formation of large particles of metal gold due to the presence of chlorine is put in evidence and can be limited by a treatment with ammonia solution. This washing procedure leads to some loss of gold with a final amount of 1.5%. Measurements of gold particle size by HRTEM are given as a function of preparation. A study of the activity in CO oxidation reaction is undertaken and correlated with TEM analysis. (C) 2004 Elsevier B.V. All rights reserved.