The ability to characterize oligosaccharides directly in complex mixtures would greatly benefit many research efforts focused on the development of biofuels from lignocellulosic biomass. We report here on the utility of chloride anion attachment in atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) tandem mass spectrometry for MW determination and structural elucidation of several mono-, di- and oligosaccharides directly without sample pretreatment or derivatization. Chloride anion forms stable adducts with these sugars upon ESI or APCI, which can be readily identified due to the chlorine isotope pattern. Fragment anions or other product anions are usually not observed, which is beneficial for mixture analysis. Upon collisional activation in MS2 experiments, these adducts readily lose HCl, which helps verify the molecular weight of each analyte. Isolating the resulting anion and subjecting it to further collision-activated dissociation experiments (MSn; n = 3-4) until no ion signal remains yields useful structural information. Examination of equimolar mixtures of mono-, di-and oligosaccharides and mixtures of enzymatically digested biomass demonstrates the ability of this methodology to ionize all the components of the mixtures without bias and with nearly equal efficiency. (C) 2012 Elsevier Ltd. All rights reserved.