Formation processes of the Pd clusters supported on USY were analyzed by means of time-resolved Quick XAFS. We found that the kind of precursors had profound effect on the formation of Pdo clusters. That is to say, Pd clusters with ca. 13 atoms were obtained through the addition of H-2 to Pd(NH3)(4)Cl-2/USY at room temperature, where the partial pressure of H-2 hardly affected the size and the rate of formation of Pd clusters. In contrast, severe aggregation occurred over PdCl2 and Pd(OAc)(2) loaded on USY. The Pd species generated on USY exhibited prominent activity in Suzuki-Miyaura reactions. The use of ammine complexes was indispensable to achieve high activity, in agreement with the Quick XAFS observations. The in situ activation (H-2 bubbling) was remarkably effective to activate the Pd/USY catalyst. These results represented that many factors including support, Pd precursor, and activation methods had profound effect on the genesis and the catalysis of Pd clusters. (C) 2009 Elsevier Inc. All rights reserved.