The polymeric complex [Ru2(CO)4(mu-OAc)2]n is the first Ru(I) catalyst that efficiently catalyzes cyclopropanation of alkenes with diazoacetic esters. The related dinuclear catalyst Ru2(CO)4(mu-OAc)2(MeCN)2 shows a similar performance only for cyclopropanation of monosubstituted alkenes.