Direct dehydration of fructose and glucose to 5-hydroxymethylfurfural (5- HMF) was studied using ionic liquids (ILs) without adding any catalysts. Various ILs were screened, and the highest 5-HMF yield of 95.6% was obtained using 1-butyl-3-methylimidazolium tosylate ([BMIM][TSO]) at 353 K for 30 min. Proton nuclear magnetic resonance (¹H NMR) spectra confirmed that the sulfonate hydrolysates of anions of [BMIM][TSO] acted as active sites for the dehydration of fructose to 5-HMF. The [BMIM][TSO] catalyzed dehydration reaction showed relatively low activation energy (Ea). A mixture of dimethyl sulfoxide and 1-sulfobutyl-3-methylimidazolium trifluoromethane sulfate (DMSO-[BSO3HMIM][OTF]) was used at 413 K for 50 min for the dehydration of glucose, which yielded 59.8% 5-HMF. The addition of t-butanol, as an isomerization promoter, to DMSO- [BSO3HMIM][OTF] led to a higher 5-HMF selectivity without sacrificing 5-HMF yield.