Regioselectively deuterium labelled precursors of defined stereochemistry are used in order to investigate the biogenesis of monoterpenoid derivatives in the essential oil of Pelargonium ssp. and Mentha x piperita. In vivo feeding experiments with subsequent headspace isolation (solid-phase micro extraction, SPME) and enantio-MDGC-MS analysis of the extracts allow to elucidate more details of the biosynthetic pathways including stereochemical aspects.