The kinetic effect of subcritical aqueous alcohols (methanol, ethanol, 1-propanol, 2-propanol, and t-butyl alcohol) on the isomerization of hexoses (mannose, glucose, and fructose) was examined at 180 degrees C at the alcohol concentrations ranging from 0 to 80% (v/v). The results showed that increase in the concentration of primary and secondary alcohols markedly promoted the aldose-to-ketose isomerization; however, the ketose was scarcely isomerized to aldose and predominantly decomposed. Further, the kinetic analysis revealed that the rate constants of isomerization and decomposition strongly depended on the dielectric constant of the aqueous alcohols. The rate constants for isomerization were more sensitive to the change in the dielectric constant than the decomposition rate constants. On the other hand, the isomerization reactions of mannose and fructose were suppressed in subcritical aqueous t-butyl alcohol despite the low dielectric constant of the solution. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.