Abstract Several deuterium-labelled secoiridoids have been prepared and tested as possible precursors for the iridoids in Fraxinus excelsior, Syringa josikaea and S. vulgaris. Oleoside 11-methyl ester was an efficient precursor for the oleosides, whereas secologanin-type iridoids gave only significant incorporation in S. josikaea. In this plant low incorporations into the oleosides were also seen for kingiside and 8-epi-kingiside. The major pathway to the oleosides therefore seems to proceed via a direct ring fission of ketologanin to oleoside 11-methyl ester. A Baeyer-Villiger-like mechanism which explains the different compounds found in the plants is proposed, and the taxonomy of the Oleaceae is discussed. Due to the unique presence of the usual pathway leading to secologanin and its congeners in Fontanesia, the Oleaceae is considered to be a member of the Gentiananae rather than Scrophulariales/Lamianae.