Ru-atoms (0.5–1.5 g) have been generated by electron bombardment and, in separate experiments, were co-condensed at 7 K with excess of 2,3-dimethylbutadiene or 1,3- or 1,4-cyclohexadiene. The complexes Ru(CO)(η4-C6H10)2 (1) and Ru(CO)(η4-C6H8)2 (2), respectively, have been isolated in 25–30% yields on warming the condensate under CO.