This work investigates the effects of Au3+/Au-0 ratio or distribution of gold oxidation states in Au/ZrO2 catalysts of different gold loadings (0.01-0.76% Au) on CO oxidation and 1,3-butadiene hydrogenation by regulating the temperature of catalyst calcination (393-673 K) and prereduction with hydrogen (473-523 K). The catalysts were prepared by deposition-precipitation and were characterized with elemental analysis, nitrogen adsorption/desorption, TEM, XPS and TPR. The catalytic data showed that the exposed metallic Au-0 atoms at the surface of Au particles were not the only catalytic sites for the two reactions, isolated Au3+ ions at the surface of ZrO2, such as those in the catalysts containing no more than 0.08% Au were more active by TOF. For 0.76% Au/ZrO2 catalysts having coexisting Au3+ and Au-0, the catalytic activity changed differently with varying the Au3+/Au-0 ratio in the two reactions. The highest activity for the CO oxidation reaction was observed over the catalyst of Au3+/Au-0 = 0.33. However, catalyst with a higher Au3+/Au-0 ratio showed always a higher activity for the hydrogenation reaction; co-existance of Au-0 with Au3+ ions lowered the catalyst activity. Moreover, the coexisting Au particles changed the product selectivity of 1,3-butadiene hydrogenation to favor the formation of more traps-2-butene and butane. It is thus suggested that for better control of the catalytic performance of Au catalyst the effect of Au3+/Au-0 ratio on catalytic reactions should be investigated in combination with the particle size effect of Au. (C) 2007 Elsevier B.V. All rights reserved.