Pt4-cluster/SiO2catalyst, prepared from the [Pt4(μ-CH3COO)8] cluster, has shown remarkably high catalytic activity with 100% selectivity to CO2 and H2 for the decomposition of formic acid as compared with the Pt-particle/SiO2 catalyst. In order to elucidate the genesis and the mechanism for catalysis by Pt4-cluster/SiO2, the samples were characterized by kinetics, Pt LIII-edge extended X-ray absorption fine structure (EXAFS), and FTIR spectroscopy. EXAFS and IR data revealed that the surface structure of the catalyst changed from the tetramer to dimers during an induction period, then to monomers which were active for the catalysis. Eventually Pt particles were formed which were of much lower catalytic activity. The structural transformations of the Pt4-cluster/SiO2 catalyst are discussed in relation to the catalytic reaction profiles.