Noncatalytic conversion of D-cellobiose (at 0.5 M) into 5-hydroxymethyl-2-furaldehyde (5-HMF), a platform chemical for fuels and synthetic materials, was analyzed at 120-200 degrees C over a wide range of water mole fraction, x(w) = 0.007-1 in a binary dimethyl sulfoxide (DMSO)-water mixture by means of the in situ C-13 NMR spectroscopy. Effects of the water content were revealed as follows: (i) The tautomerization of the anomeric residue of D-cellobiose from 6-glucose to D-fructose type was not initially observed at a lower water content, in contrast to the significant tautomerization into the 6-fructose type in a higher water content and pure water. (ii) The lower the water content, the faster the glycosidic-bond cleavage by hydrolysis, because of the high reactivity of solitary water molecules with the large partial charges more naked as in supercritical water clusters due to the isolation by the organic solvent DMSO. (iii) The amount of 6-fructose as the intermediate product was larger at the higher x(w); despite the increase of 6-fructose, the production of 5-HMF from 6-fructose was slowed down. (iv) A high 5-HMF yield of 71% was reached at x(w) = 0.20-0.30 that was 6-10 times the initial D-cellobiose concentration. The best yield of 5-HMF was attained in the low x(w) when the polymerization paths into NMR-undetectable species via 5-HMF and anhydromonosaccharides were effectively suppressed. This study provides a new framework to design optimal, noncatalytic reaction process to produce 5-HMF from cellulosic biomass by tuning the water content as well as the temperature and the reaction time.