Abstract The biosynthesis of the oleoside-type of glucosides (oleosides) has been investigated in two species of Oleaceae, namely Fraxinus excelsior and Syringa josikaea. Deuterated analogues of deoxyloganic acid, 8-epi-deoxyloganic acid, 7-epi-loganic acid and loganic acid were prepared. On administration of the above compounds high incorporations of deoxyloganic acid as well as of loganic acid and its 7-epimer were obtained, strongly implying that 7-ketologanic acid is an intermediate in the biosynthesis of oleosides. The incorporations were measured by 2H NMR in several isolated secoiridoids all belonging to the oleosides. In addition, the content of acidic iridoids (both carbocyclic and seco-type) in two additional species of Oleaceae, namely Ligustrum ovalifolium and Phillyrea latifolia, were examined. Based on these results, a plausible biosynthetic route from deoxyloganic acid, 7-epi-loganic acid, 7-ketologanic acid, 8-epi-kingisidic acid, oleoside 11-methyl ester, 7-β-1- d-glucopyranosyl 11-methyl oleoside and ligstroside to oleuropein can now be presented.