The adsorption of Co-2(CO)(8) onto the dehydrated Y-faujasite powder under an N-2 atmosphere and onto the tetrahydrofuran slurry of Y-faujasite under a mixed CO and H-2 atmosphere predominately yielded supported Co-4(CO)(12) and supported Co-6(CO)(16), respectively. The molecular cobalt-carbonyl clusters and their decarbonylated products have been structurally characterized by extended X-ray absorption. ne structure (EXAFS). The decarbonylated sample a possesses a cluster of two Co atoms and the decarbonylated sample b has a cluster phase of three Co atoms. The decarbonylated sample a exhibited higher CH4 conversion and C2+ selectivity (C2+ selectivity = SigmanC(n)(n = 2 -5)/SigmanC(n) ( n = 1 - 5) * 100%) in comparison with the decarbonylated sample b in methane homologation. A density functional theory (DFT) model was employed to calculate Co clusters adsorbed on a silica substrate which simulates Y-faujasite encapsulated Co clusters. The structural geometries, net spin electronic charge densities, energies of the metal - silica and metal - metal interactions in stable geometries are discussed and used to interpret the cluster size dependence of the catalytic activity and selectivity to C2+ hydrocarbons in the methane homologation.