The metabolism of taxanes by human liver microsomes is regioselective: the major metabolite of paclitaxel formed by CYP2C8 results from the hydroxylation on the taxane ring at C-6. Hydroxylations on the lateral chain at C-13 are catalyzed by CYP3A4 on either the tert-butyl of docetaxel or the C-3’ phenyl of paclitaxel. Furthermore, the presence of the acetyl group in position 10, has been shown to play an important role in determining the regioselective oxidation by CYP. The biotransformation of a series of docetaxel analogues by human liver microsomes and recombinant CYP expressed in HEK293 cells was examined by high-performance liquid chromatography/mass spectrometry. The formation of derivatives was lost when the tert-butyl of docetaxel was replaced by an ethyl group. Addition of an aliphatic chain at either position 7 or 10 led to the formation of oxidized metabolites, whereas addition at both 7 and 10 totally impaired the production of derivatives. Similarly, the insertion of a phenylbenzoyl group in position 10 prevented the biotransformation of the molecule. The site of hydroxylation could be clearly located on the aliphatic chain inserted in position 10, but remained on the tert-butyl at C-13 when the aliphatic chain was added in position 7. These reactions were catalyzed only by CYP3A4: recombinant CYP3A4 generated the same metabolites as liver microsomes did, whereas neither CYP3A5 nor 2C8 or 2C9 could form derivatives. In conclusion, the presence of substituents in position 10 of the docetaxel molecule plays a pivotal role in determining the site of oxidation by CYP3A4.