IntroductionFlavonoid glycosides are predominant forms of naturally occurring flavonoids in plants, representing a large group of secondary plant metabolites [1][2][3]. They all contain a C 15 flavonoid as an aglycone, are usually divided into O-and C-glycosyl flavonoids, are of interest because they have biological activities, are useful for chemotaxonomy, and are used as tracers in medicinal plant preparations. Recently, they have also received considerable interest in foodstuffs and nutraceuticals because of their antioxidant and anticancer properties [4].During the last several years our research at the University of Antwerp has focused on the structural analysis of O-, C-and O,C-diglycosides isolated from medicinal plants using fast atom bombardment (FAB) or electrospray (ES) ionization in combination with collision-induced dissociation (CID) and tandem mass spectrometry (MS/MS) [5][6][7][8][9][10]. In this article we will review various tandem mass spectral approaches to the structural characterization of flavonoid glycosides. With regard to the structure elucidation of flavonoid O-diglycosides it will be illustrated how structural information can be obtained on (i) the carbohydrate sequence, (ii) the aglycone part, and (iii) the linkage in the diglycosidic part. Tandem mass spectral analyses have mainly been performed in the positive ion mode on protonated molecules generated by FAB or ES. In more recent work, however, which is briefly reported in the 888 ANALUSIS, 2000, 28, N°10