Negative-ion electrospray mass spectrometry (ES-MS) with collision-induced dissociation (CID) and MS/MS scanning on a quadrupole-orthoganal time-of-flight instrument provide a sensitive means for structural analysis of neutral underivatized oligosaccharides. Molecular mass information can be readily obtained from the dominant [M - H]- ions in the ES mass spectrum formed with subnanomole amounts of oligosaccharides, and similar sensitivity is available from CID-MS/MS to give structural details. The CID spectra of 14 oligosaccharides demonstrated that sequence and partial linkage information can be derived and isomeric structures can be differentiated. Series of C-type fragment ions give sequence information while the double glycosidic D-type cleavage of a 3-linked GlcNAc or Glc and the saccharide ring fragmentation of the 0,2A-type from 4-linked GlcNAc or Glc can provide partial linkage information. The distinctive D- and A-cleavages are important for differentiation of oligosaccharide type 1 and type 2 chains and to define the blood group H, Le(a), Le(x), Le(b), and Le(y) determinants carried by their fucosylated analogues.