Lipoxygenases (LOX) catalyse allylic oxidations and epoxidations of a co-substrate in the presence of an unsaturated fatty acid containing a 1,4-pentadiene moiety. One- and two-step enzyme assays were established to verify the role or involvement of LOX in such co-oxidations. It was shown that LOX is only involved in the formation of reactive hydroperoxides, but not in the oxidation of a co-substrate, assuming a mechanism involving free peroxy radicals for the latter. Ten mono- and sesquiterpenes were used as co-substrates and the resulting products were analysed by mass spectrometry. A semi-preparative approach was developed using (+)-valencene as an example, and the resulting products were isolated by preparative GC and their structures elucidated by NMR spectroscopy.