Silibinin is the main isomer of a group of flavanoids extracted from the seeds of the milk thistle weed, a common herb that is widely used to maintain liver health and for the treatment of liver disorders. Silibinin when incubated with human liver microsomes produced one major metabolite and at least two minor metabolites. Tandem mass spectrometry (MS) was used to identify the metabolite structures partially. MS studies confirmed that the major metabolite is demethylated silibinin and the two minor metabolites are mono-hydroxy and di-hydroxy silibinin. The K(m) value for the demethylation shows that silibinin has a strong affinity for the cytochrome P450 enzymes.