Gold used to be considered to have no catalytic activity. In the 1980s, however, Masatake Haruta found that nano-sized gold particles supported by metal oxides can catalyze the oxidation of carbon monoxide. This work examines the oxidation of carbon monoxide (CO) and the adsorption/desorption behaviors on nano-sized gold catalyst at room temperature by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Carbonate, bicarbonate and carboxylate were observed. The effects of various factors (relative humidity, CO gas concentration, and total surface area) on the CO conversion efficiency were studied using the response surface designs in the Experiment Design Method. The results indicate that the conversion efficiency of CO was high when the ratio of CO and O-2 was close to 1: 1. The gas concentration is the most important factor, followed by the weight of gold catalyst, followed by relative humidity. An appropriate humidity enhances the catalytic reaction in the long-term. (C) 2008 Elsevier B.V. All rights reserved.