The complexes H2Ru3(CO)9C8H12, HRu3(CO)9C8H11, Ru4(CO)11C8H10, Ru4(CO)12C8H10, and Ru4(CO)12C8H12 have been identified as the products of the reaction of α-H4Ru4(CO)12 with either cyclo-octa-1,5-diene or cyclo-octa-1,3-diene. The complexes are also formed by reaction of α-H2Ru4(CO)13 with cyclo-octa-1,5-diene, and the complexes H2M3(CO)9C8H12(M = Ru or Os) are obtained by reaction of M3(CO)12 with cyclo-octene. Structures of the complexes are discussed, where possible, in terms of spectroscopic data and in the case of Ru4CO11C8H10 on the basis of single crystal X-ray analysis. 1H N.m.r. studies of H2Ru3(CO)9C8H12 indicate that the molecule undergoes two distinct types of fluxional behaviour involving in the first instance the C8H12 ligand, and in the second the two hydrido-ligands. ΔG‡ Values for these processes (and for the corresponding osmium complex) have been determined. HRu3CO9C8H11 reacts with hydrogen gas to give (H2Ru3CO)9C8H12. The complexes Ru4(CO)11C8H10 and Ru4(CO)12C8H12 react with hydrogen to give H2Ru3(CO)9C8H12 and α-H4Ru4(CO)12, and Ru4(CO)12C8H10 forms α-H4Ru4(CO)12 only. )α-H4Ru4(CO)12 is converted into α-H2Ru4(CO)13 in ca. 30% yield by reaction with cycloheptene. The compounds Ru4(CO)12C8H10, Ru4(CO)12C8H12, HRu3-(CO)9C8H11, and H2Ru3(CO)9C8H10 have been obtained from the reaction of Ru3(CO)12 with cyclo-octa-1,3-diene.