Sesquiterpenes like germacrene D are naturally occurring in plants, bacteria, fungi, and marine invertebrates. They are important flavor compounds and may be active as antibiotics, insect repellents, attractants, or pheromones or may be produced in response to attacking herbivores. Sesquiterpene synthases (cyclases) catalyze the biosynthesis of sesquiterpenes from the ubiquitous precursor farnesyl diphosphate 1 (FDP, Scheme 1). Most sesquiterpenes are chiral molecules, but usually only one of the two possible enantiomers of a sesquiterpene is produced in a single species. Solidago canadensis is an exception to this rule, because it contains the sesquiterpene enantiomers (+)-germacrene D 2a and (-)-germacrene D 2b in approximately equal amounts. We have isolated two different sesquiterpene synthases from S. canadensis: (+)germacrene D synthase and (-)-germacrene D synthase. Both synthases catalyze the formation of enantiomerically pure products. After characterizing these synthases biochemically, we studied the mechanisms of the formation of the germacrene D enantiomers. To that end, we incubated both synthases with their substrate trans,trans-farnesyl diphosphate 1 carrying deuterium labels in different positions. Incubation products were analyzed using mass spectrometric techniques. The action of (-)-germacrene D synthase involves a 1,3-hydride shift, whereas (+)-germacrene D is formed via a double 1,2-hydride shift.