The ability of ruthenium(I) triazenide complexes [Ru(CO)3(ArNNNAr)]2 (Ar = C6H4-4-X, X = CH3, Cl, Br) to catalyze the cyclopropanation of alkenes with methyl diazoacetate is investigated. With terminal alkenes (styrene, ethyl vinyl ether, 1-hexene), the cyclopropanecarboxylic esters are formed in good to high yield and with an E : Z diastereoisomer ratio of about 1.0 in most cases. 2-Methyl-2-butene is cyclopropanated in low yield but with a syn-selectivity up to 90:10.