The quick EXAFS technique was used to study the formation and growth of bimetallic PtRh particles in zeolite NaY during reduction. Before reduction, only metal-oxygen signals were detectable. While increasing the temperature at a rate of 5 degrees C/min and passing H-2 through the in situ cell, 1 min EXAFS scans were recorded at the Ph and Pt edges. At 40 degrees C, an intense Ph-Ph signal suddenly appeared, indicating the formation of small monometallic Ph clusters. The remaining Rh-O signal slowly decreased between 100 and 200 degrees C, simultaneous with the appearance and slow growth of a small Ph-Pt signal. The Pt edge EXAFS changed less abruptly than the Ph edge EXAFS. Above 80 degrees C, the Pt-O signal started to decrease, and at 100 degrees C Pt-Pt and Pt-Ph signals appeared simultaneously, pointing to the formation of small bimetallic particles. These signals grew slowly until 300 degrees C, even after the Pt-O signal had disappeared. This indicates that small bimetallic particles move through the channels of the zeolite and merge to form larger particles. This merging should also involve the big monometallic Ph clusters formed at 40 degrees C, probably situated in the supercages of the zeolite. At the end of the reduction, two kinds of clusters were present : one having a Ph core, possibly also completely monometallic, the other consisting of Pt-rich bimetallic PtRh clusters. The different reduction behavior of Ph and Pt leads to a nonhomogeneous composition of the metal particles, although they all have a similar size.