Catalysis and structures of supported ruthenium carbido clusters [Ru6C(CO)16/(CH3)]-/oxides were investigated in comparison with those of supported non-carbido clusters prepared from [Ru-6(CO)(18)](2-) and traditional ruthenium catalysts prepared from Ru(NO)(NO3)(3). Oxygenate synthesis (methanol, dimethyl ether, and formaldehyde) in CO/H-2 reaction was observed on the supported carbido clusters in contrast to the preferential formation of methane and hydrocarbons on the conventional Ru catalysts and the supported non-carbido clusters. The active structure for oxygenate synthesis crucially depended on the kind of oxides; on basic oxides (MgO and La2O3) the cluster of framework was incorporated with surface oxygen atoms and expanded or shrunk under CO/H-2 reaction conditions depending on the CO pressure. The reversible expansion-shrinking of the cluster framework was correlated with the activated CO adsorption (CO breathing-induced structural change). On TiO2 the [Ru6C] framework always held a shrunk structure as proved by EXAFS. The expanded clusters showed high selectivities in oxygenate synthesis. The shrunk [Ru6C]/TiO2 also showed much higher activity than the non-carbido cluster or the traditional catalyst. IR,and hydrogen isotope effects suggested the formation of oxygenates through a mu 2-formyl intermediate. The switchover of reaction path from the formation of methane and hydrocarbons to oxygenate synthesis is ascribed to the interstitial carbido carbon which has structural effect like a central spring and electronic effect as a four-electron donor on the behavior of the cluster framework.