The one-pot hydrolysis/dehydration of mono/di/polysaccharides into furfural (FUR) or 5-hydroxymethyl-2-furaldehyde (HMF) in the presence of an acidic ionic liquid, 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO4]), at 100°C, was investigated and the results were compared with those using 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), with or without CrCl3 or H2SO4. The ionic liquid [EMIM][HSO4] is effective in converting xylose and fructose or related polysaccharides into FUR (e.g. from xylose: 84% yield at 6h) or HMF (e.g. from fructose: 88% yield at 30min), but not glucose and polymers containing these units, which contrasts to that observed for the [BMIM][Cl]/CrCl3 system. The latter is quite effective in converting glucose and related disaccharides into HMF, but not the polysaccharides cellulose and starch. For the latter feedstocks, the addition of H2SO4 or [EMIM][HSO4] to cellulose/[BMIM][Cl]/CrCl3 enhances HMF yields, presumably by accelerating the hydrolysis step, although the selectivity of the dehydration of glucose monomers seems rather poor (HMF yield