Au/zeolite catalysts prepared with a deposition-precipitation method were characterized with quick XAFS (QXAFS) in combination with IR. The data were correlated with the catalytic performance in the CO-O(2) reaction conducted at 273 K. On the basis of the XANES analysis of Au loaded on H-Y, the deposited Au(2)O(3) was observed at the initial stage. The transformation of Au(2)O(3) to form metal Au clusters was observed at 473 K in a H(2) atmosphere. The fact was supported by the IR measurement of adsorbed CO and the subsequent reaction with O(2). Detailed clustering process of Au supported catalysts could be directly followed by EXAFS analysis. The growth of metal Au proceeded via the formation of a Au(55) cluster at 473 K. Then it agglomerated to give metal Au with diameter of 2 nm at 723 K. The addition of H(2) was effective to retard the sintering of Au clusters. A similar phenomenon was observed over Au loaded on USY zeolite. In marked contrast to the H-Y and USY supports, significantly agglomerated Au particles generated on Na-Y zeolite, indicating the importance of the presence of acid sites in keeping the Au clusters with highly dispersed form. The performance of 5 wt % Au loaded on H-Y and USY in the CO-O(2) reaction was remarkably sensitive to the pretreatment temperature and the gas atmosphere. The catalyst pretreated with hydrogen showed a two-spike pattern with respect to the pretreatment temperature. Namely, the optimum activity was observed after the pretreatment at 373 and 723 K, where the temperatures corresponded to the generation of Au(2)O(3) and metal Au clusters with 2 nm diameter as evidenced by QXAFS analysis, respectively. The reason for enhancement of the activity of Au/H-Y by the addition of H(2) in the pretreatment step could be attributed to the formation of metal Au with appropriate size. In contrast to the H-Y and USY support, Au loaded on Na-Y prepared under the same condition was almost inactive in the reaction due to the formation of aggregated metal Au.