[Rh-12(CO)(30)](2-) and [Rh-5(CO)(15)](-) were prepared on MgO and on gamma-Al2O3, each by surface-mediated synthesis from chemisorbed [Rh(CO)(2)(acac)] in the presence of CO. [Rh-12(CO)(30)](2-) was formed when the adsorption of [Rh(CO)(2)(acac)] onto either support was carried out in the presence of dehydrated hexane, whereas [Rh-5(CO)(15)](-) was formed when the hexane contained water. Either cluster anion could be extracted into methanol solution by cation metathesis with [K][CH3COO]; [Rh-12(CO)(30)](2-) was extracted under N-2, and [Rh-5(CO)(15)](-) was extracted under CO. The organometallic chemistry on the MgO and gamma-Al2O3 surfaces is similar to that occurring in basic solution. The surface-mediated synthesis is an efficient method for the preparation of [Rh-5(CO)(15)](-) (which was obtained in 47% yield); this anion in solution is easily converted into [Rh-12(CO)(30)](2-).