The properties of the Δ6 desaturase/acetylenase from the moss Ceratodon purpureus and the Δ12 acetylenase from the dicot Crepis alpina were studied by expressing the encoding genes in Arabidopsis thaliana and Saccharomyces cerevisiae. The acetylenase from C. alpinaΔ12 desaturated both oleate and linoleate with about equal efficiency. The desaturation of oleate gave rise to 9(Z),12(E)- and 9(Z),12(Z)-octadecadienoates in a ratio of approximately 3: 1. Experiments using stereospecifically deuterated oleates showed that the pro-R hydrogen atoms were removed from C-12 and C-13 in the introduction of the 12(Z) double bond, whereas the pro-R and pro-S hydrogen atoms were removed from these carbons during the formation of the 12(E) double bond. The results suggested that the Δ12 acetylenase could accommodate oleate having either a cisoid or transoid conformation of the C12-C13 single bond, and that these conformers served as precursors of the 12(Z) and 12(E) double bonds, respectively. However, only the 9(Z),12(Z)-octadecadienoate isomer could be further desaturated to 9(Z)-octadecen-12-ynoate (crepenynate) by the enzyme. The evolutionarily closely related Δ12 epoxygenase from Crepis palaestina had only weak desaturase activity but could also produce 9(Z),12(E)-octadecadienoate from oleate. The Δ6 acetylenase/desaturase from C. purpureus, on the other hand, produced only the 6(Z) isomers using C16 and C18 acyl groups possessing a Δ9 double bond as substrates. The Δ6 double bond was efficiently further converted to an acetylenic bond by a second round of desaturation but only if the acyl substrate had a Δ12 double bond and that this was in the Z configuration.