Dealuminated zeolite Y was used as a crystalline support for a mononuclear ruthenium complex synthesized from cis-Ru(acac)(2)(C2H4)(2). Infrared (IR) and extended X-ray absorption fine structure spectra indicated that the surface species were mononuclear ruthenium complexes, Ru(acac)(C2H4)(2)(2+), tightly bonded to the surface by two Ru-O bonds at Al3+ Sites of the zeolite. The maximum loading of the anchored ruthenium complexes was one complex per two Al3+ sites; at higher loadings, some of the cisRu(acac)(2)(C2H4)(2) was physisorbed. In the presence of ethylene and H-2, the surface-bound species entered into a catalytic cycle for ethylene dimerization and operated stably. IR data showed that at the start of the catalytic reaction, the acac ligand of the Ru(acac)(C2H4)(2)(2+) species was dissociated and captured by an Al3+ site. Ethylene dimerization proceeded similar to 600 times faster with a cofeed of ethylene and H2 than without H-2. These results provide evidence of the importance of the cooperation of the Al3+ sites in the zeolite and the H-2 in the feed for the genesis of the catalytically active species. The results presented here demonstrate the usefulness of dealuminated zeolite Y as a nearly uniform support that allows precise synthesis of supported catalysts and detailed elucidation of their structures.