The hydrogenolysis of the acetato group by molecular hydrogen in phosphine-substituted ruthenium(I) and (II) carbonyl carboxylates has been reinvestigated in the presence of Na 2 CO 3 at various temperatures. From mononuclear complexes such as Ru(CO) 2 (MeCO 2 ) 2 (PR 3 ) 2 , the intermediate hydrido-acetato complex HRu(CO) 2 (MeCO 2 )(PR 3 ) 2 has been identified among the products formed working at low temperature. In the case of the binuclear complexes Ru 2 (CO) 4 (MeCO 2 ) 2 (PR 3 ) 2 the products obtained at the lowest reaction temperature are H 4 Ru 4 (CO) 9 (PR 3 ) 3 , H 4 Ru 4 (CO) 8 (PR 3 ) 4 and H 2 Ru(CO) 2 (PR 3 ) 2 . The complex H 4 Ru 4 (CO) 9 (PR 3 ) 3 is formed from H 4 Ru 4 (CO) 8 (PR 3 ) 4 and CO produced from carbon dioxide by the reverse water gas shift reaction catalysed by ruthenium complexes:Carbon dioxide arises from the reaction between Na 2 CO 3 and acetic acid. Several tests support the involvement of the ruthenium-catalysed reverse water gas shift reaction. At higher temperatures phosphido ruthenium complexes are formed. The same products are obtained in the absence of carbon dioxide.