The CO oxidation reaction has been studied on Au/TiO2 catalysts prepared by incipient wetness (IW) and deposition-precipitation (DP) methods, with a range of metal loadings from 0.1 to 5 wt% for the former. We have found that there are three types of CO2 production from fresh catalysts which have been calcined to only 120 degreesC, namely noncatalytic (types I and II) and catalytic (type III). The DP catalysts are far more active for type III reaction, showing activity at room temperature, whereas the IW catalysts generally do not show steady-state conversion until > 120 degreesC. However, IW catalysts with low loadings produce type I CO2 (that formed at room temperature), whereas higher loadings produce only type II (that formed when heating), with intermediate loadings showing both. We associate type I CO2 production with monolayer Au catalysts, which have intimate contact with the TiO2, and are probably already in some oxidic form. Type II behavior we associate with three-dimensional oxidic An, the surface of which is no longer in direct contact with the support. The poor performance of IW catalysts compared with DP catalysts is ascribed to the presence of chloride in the former, which induces a larger average particle size, and vulnerability to extensive sintering. (C) 2003 Elsevier Inc. All rights reserved.