Flavonoids are important polyphenolic substances with widespread occurrence in plants and therefore in the human diet. Although considerable work has been done on the pharmacology of flavonoids, the understanding of their metabolism is still incomplete. In this work, the in vitro glucuronidation of the common dietary flavonoids quercetin and kaempferol by human UDP-glucuronosyltransferase microsomes (UGT-1A9) was investigated using HPLC and LC-MS. The two flavonoids were extensively metabolised by this enzyme with four monoglucuronides of quercetin and two of kaempferol being detected after incubation. The presence of a quercetin monoglucuronide in the urine of a volunteer after consumption of Ginkgo biloba tablets was demonstrated.