The conversion of D-glucose to 5-hydroxymethylfurfural (HMF) in the presence of 5.48 mol% 1-(1-propylsulfonic)-3-methylimidazolium chloride acidic ionic liquid catalyst in DMSO at 150°C was studied using (1)H, (13)C NMR, and visible spectroscopy. The HMF yield rapidly increases in the first 100 min of reaction, however yield drops beyond 100 min and levels off to a maximum yield of about 15.7% around 600 min. The visible spectroscopy study of the reaction mixture suggests that rate of HMF formation slows down after 100 min due to increase in the rate of humin formation after first 100 min. A mechanism has been proposed and key intermediates in the pathway could be identified by studying the (13)C NMR spectra of acidic ionic liquid catalyzed transformations of C-1 and C-2 (13)C labeled d-glucose under identical conditions. The proposed mechanism involves the isomerization of d-glucose to d-fructose via the complexation of the open chain sugar with the imidazolium cation of the acidic ionic liquid catalyst.