Supported Au catalysts Au-Au+-Cl-x/Fe(OH)(y) (x < 4, y <= 3) and Au-Cl-x/Fe2O3 prepared with co-precipitation without any washing to remove Cl- and without calcining or calcined at 400 degrees C were studied. It was found that the presence of Cl- had little impact on the activity over the unwashed and uncalcined catalysts; however, the activity for CO oxidation would be greatly reduced only after Au-Au+-Cl-x/Fe(OH)(y) was further calcined at elevated temperatures, such as 400 degrees C. XPS investigation showed that Au in catalyst without calcining was composed of Au and Au+, while after calcined at 400 C it reduced to Au-0 completely. It also showed that catalysts precipitated at 70 degrees C could form more Au+ species than that precipitated at room temperatures. Results of XRD and TEM characterizations indicated that without calcining not only the Au nano-particles but also the supports were highly dispersed, while calcined at 400 degrees C, the ALI nano-particles aggregated and the supports changed to lump sinter. Results of UV-vis observation showed that the Fe(NO3)(3) and HAuCl4 hydrolyzed partially to form Fe(OH)3 and [AuClx(OH)(4-x)](-) (X = 1-3), respectively. at 70 C, and such pre-partially hydrolyzed iron and gold species and the possible interaction between them during the hydrolysis may be favorable for the formation of more active precursor and to avoid the formation of Au-Cl bonds. Results of computer simulation showed that the reaction molecular of CO or O-2 were more easily adsorbed on Au+ and Au-0, but was very difficultly absorbed on Au-. It also indicated that when Cl-was adsorbed on Au-0, the Au atom would mostly take a negative electric charge, which would restrain the adsorption of the reaction molecular severely and restrain the subsequent reactions while when Cl- was adsorbed on Au+ there only a little of the Au atom take negative electric charge, which resulting a little impact on the activity. (c) 2006 Elsevier B.V. All rights reserved.