The structure of attached Rh-4(CO)(12) on tris(hydroxymethyl)phosphine (P(CH2OH)(3),THP)-modified silica was investigated by EXAFS and IR spectroscopy, The catalytic behavior of the Rh-4 carbonyl cluster coordinated with THP ligands anchored on SiO2(P(CH2OH)(3-x)(CH2OSi)(x) (x = 1, 2)) in gas-phase hydroformylation reactions was investigated, The number of THP ligands coordinating to the Rh-4 cluster depended on the loading of THP. Rh-4 clusters on SiO2 loaded with 6.2 wt% THP, on which the average distance of P atoms of the neighboring THP was 8 Angstrom, were coordinated by two THP ligands, and the Rh-4 clusters were distorted. Rh-4 clusters on SiO2 loaded with 1.6 wt% THP, on the other hand, were coordinated by one THP ligand and the clusters were not distorted, The attached cluster was active for olefin hydroformylation reaction, The reaction rate of hydroformylation on this catalyst was comparable to that on Wilkinson catalyst in solution. The distorted Rh-4 framework, which is coordinated by two THP, was much more active in hydroformylation reactions than was that of monosubstituted clusters of THP/SiO2 or Rh-4(CO)(10)(THP)(2) in solution. In situ IR spectroscopy and EXAFS revealed that a reversible coordination of CO ligands attached to the Rh-4 cluster was associated with the selective hydroformylation reaction. In the hydroformylation atmosphere, the Rh-4 cluster is almost saturated by CO ligands and effectively suppressed the hydrogenation reaction as a side reaction of hydroformylation. In contrast, under the reaction condition of hydrogenation, CO ligands were removed to increase the reaction rate of hydrogenation. The reversible CO bonding in the surface-grafted Rh-4 site was related to the dynamic control of selective hydroformylation of olefins.