Positive ion fast atom bombardment mass spectrometry appears to be a very useful method for the determination of molecular weight and composition of underivatized di- and trisaccharides. Information on the identity of the monosaccharide units can be obtained from the metastable ion and collisional activation spectra of selected ions. The type of linkage between the monosaccharides is reflected in some spectral characteristics, but the differences are relatively small and do not always allow an unambiguous identification. The position of a fructose unit in a trisaccharide molecule is shown by the collisional activation spectra of the [M + H]+ ion, as an anhydrofructose molecule is easily eliminated from the ions in which fructose is in a terminal position.