The probable structures of the inclusion complexes of beta-cyclodextrin (beta-CD) with (+)-catechin (CA) and (-)-epicatechin (EC) in D2O at 35 degrees C were investigated using NMR. In the beta-CD . CA inclusion complex, a large portion of the flavonoid skeleton was included in the beta-CD cavity and the axis of C-1'-C-4' of the B ring inclined to the molecular axis of beta-CD, and the CA molecule fitted tightly with beta-CD. In the beta-CD EC inclusion complex, the B ring was included deeply in the beta-CD cavity from the secondary hydroxyl group side and the A ring was left outside the cavity near the secondary hydroxyl group side and EC molecule fitted loosely.