The reaction of triruthenium dodecacarbonyi. [ Ru,(CO),,], with water leads to the formation of the tetranuclear hydrides cc-[Ru4(CO),,H4] and a-[Ru,(CO)~,H~]. The significance of this reaction to the possible activation of alkanes is discussed. The corresponding reaction of [Os,(CO>,,] leads to the isolation of clusters based on three, four, five, six, and seven metal atoms, viz. [Os,(CO),,H(OH)]. [OS~(CO)~,H,]. [OS,(CO),~H~], [ O S ~( C O ) ~~H ~' J , [OS~(CO)~~H,], [OS~(CO)~~H,], and [OS,C(CO)~,H,].The probable structures of these polynuclear osmium carbonyls are discussed and compared with those of the previously characterized osmium cluster carbonyls.