A new anion dopant for oligosaccharides is developed for use in matrix-assisted laser desorption/ionization mass spectrometry. Two types of sulfate-attached quasimolecular ions are formed in the negative ion mode when neutral oligosaccharides are doped with dilute H2SO4 solutions. Under mild conditions, i.e., low H2SO4 concentration (approximately 10(-3) M) and threshold laser fluence, a sulfate adduct [M + HSO4]- is formed. With more concentrated H2SO4 solutions (approximately 10(-2) M) and higher laser fluence, in situ derivatization of the oligosaccharides occurs to produce an ion whose m/z corresponds to a sulfate derivative [M + HSO4 - H2O]-. Hydrogen sulfate appears to be a general anion dopant because it forms complexes with a wide variety of neutral oligosaccharides. Conversely, anionic oligosaccharides form neither the adduct nor the derivative. The combination of complex formation (with neutral oligosaccharides) and the deprotonation of acidic oligosaccharides allows simultaneous detection of the respective mixture.