The development of catalyst-controlled stereoselective olefin metathesis processes1has been a pivotal recent advance in chemistry. The incorporation of appropriate ligands within complexes based on molybdenum2, tungsten3and ruthenium4has led to reactivity and selectivity levels that were previously inaccessible. Here we show that molybdenum monoaryloxide chloride complexes furnish higher-energy (Z) isomers of trifluoromethyl-substituted alkenes through cross-metathesis reactions with the commercially available, inexpensive and typically inertZ-1,1,1,4,4,4-hexafluoro-2-butene. Furthermore, otherwise inefficient and non-stereoselective transformations withZ-1,2-dichloroethene and 1,2-dibromoethene can be effected with substantially improved efficiency andZselectivity. The use of such molybdenum monoaryloxide chloride complexes enables the synthesis of representative biologically active molecules and trifluoromethyl analogues of medicinally relevant compounds. The origins of the activity and selectivity levels observed, which contradict previously proposed principles5, are elucidated with the aid of density functional theory calculations.