The interaction of ruthenium carbonyl, Ru3(CO)12 with rare earth oxides of high surface area, >50m2g-1, has been studied. [Ru3(μ H)(CO)10(μ-0M=)] is formed on holmia, but on lanthana only [Ru(CO)2]n species are observed. Reduction of the carbonyl ligands takes place at <573K to give catalysts for the hydrogenation of carbon monoxide with activity and selectivity dependent on the particular rare earth oxide and pretreatment. Over ceria, the product is up to 55 wt% C2-5 olefins. A similar selectivity is obtained over lanthana only after redispersion through a reduction-oxidation-reduction cycle.