The detailed structure of Ag Clusters formed in H zeolites during H-2 reduction as well as under NO + C3H8 + O-2 + H-2 reaction condition is examined with H-2-TPR, XRD, UV-vis, and Ag K-edge XAFS. H-2-TPR results suggest a quantitative conversion of the exchanged Ag+ ion to Ag-2p(p+) cluster in Ag-MFI during, H-2 reduction in a temperature range from 373 to 573 K. Combined with the UV-vis spectrum of the . UV-vis and cluster, exhibiting bands at 255 and 305 nm, and Ag K-edge EXAFS result, the structure of the cluster is identified as Ag-4(2+). UV-vis and K-edge XANES/EXAFS results reveal that, during NO + C3H8 + O-2 + H-2 reaction at 573 K, part of the Ag+ ions are converte Ag-n(delta+) clusters, whose average Structure can be close to Ag-4(2+). The amount of the clusters increases with the Ag/Al ratio of Ag zeolites. The NO reduction rate of Ag-MFI for the NO + C3H8 + O-2 reaction is significantly improved by an addition of 0.5% H-2, and the rate increases with Ag/Al ratio of Ag-MFI. This Cluster does not form on Ag-MOR under the same conditions, and the NO reduction rate does not increase by H-2 addition. The structure-activity relationship shows that the Ag-n(delta+) probably the Ag-4(2+) Cluster, in Ag-MFI catalysts is responsible for the selective reduction of NO by C3H8. (C) 2004 Elsevier Inc. All rights reserved.