Abstract The mechanism of the biological hydroxylation of the isopropenyl side-chain of the sesquiterpene capsidiol, to give 13-hydroxycapsidiol, has been investigated with 13C-NMR techniques, using capsidiol biogenetically enriched with 13C from [1,2-13C2] acetate. The results indicate that neither an allylic rearrangement nor the formation of an epoxide intervene in the process.