[Os-3(CO)(12)], [H4Os4(CO)(12)], [H2Os4(CO)(12)](2-), [Os5C(CO)(14)](2-), and [Os10C(CO)(24)](2-) have been synthesized selectively and in high yields by reductive carbonylation or hydrogenation of OsCl3 or alpha-[Os(CO)(3)Cl-2](2) supported on silica in the presence of alkali carbonates. The selectivity of the reaction is controlled by the choice of (i) the nature and quantity of the alkali carbonate (Na2CO3 or K2CO3) added to silica, (ii) temperature, (iii) reaction time, and (iv) the gas-phase composition (CO, CO + H-2, or H-2). These surface-mediated syntheses are often more selective and more efficient and usually require less drastic conditions than the best known syntheses in solution, confirming the potential use of the silica surface as a new reaction medium to prepare both neutral and anionic metal carbonyl clusters.