Mannose, fructose and glucose were treated in subcritical water at 220°C, and the disappearance of each substrate and formation of other hexoses were observed at various residence times in a tubular reactor. Isomerization of mannose to fructose occurred most frequently, followed by mannose to glucose; no isomerization of glucose to mannose was observed. The temperature dependencies of mannose disappearance, the isomerization of mannose to fructose and glucose, and the resulting degradation of the produced fructose and glucose were observed in the range of 180 to 240°C. The activation energy and frequency factor for each process were evaluated according to the Arrhenius equation, and it was found that the enthalpy-entropy compensation for the processes was maintained.