Structural information on oligosaccharides was obtained by post-source decay matrix-assisted laser desorption/ionization mass spectrometry. A systematic study of derivatized oligosaccharides showed that products relatively aminated with benzylamine allow the formation of [M+H]+ molecular species. Metastable protonated species decompose in the field-free region (post-source decay) to yield predictable and reproducible fragmentation patterns. Information on sugar sequence and branching can be derived from such mass spectra.