Interaction of cyclododeca-1,5,9-triene (CDT) isomers with Ru3(CO)12 gives four complexes, characterised as HRu3(CO)9(C12H15), HRu3(CO)9(C12H17), HRu3(CO)7(C12Hn)2(n probably 17), and Ru4(CO)10(C12H16). Spectroscopic properties of the complexes are described, as are some reactions with phosphorus donor ligands. The results are discussed in terms of the structures of the complexes, two of which had been determined previously by X-ray methods.