Our group studied the decomposition behavior of 5-hydroxymethylfurfural (5-HMF) in both subcritical and supercritical water. The experiments were performed in the temperature range of 175-450 degrees C at 25 MPa, with residence times ranging from 80 to 400 s. The decomposition of 5-HMF had Arrhenius characteristics, and the liquid product acted as an intermediate compound to form gas products of mainly CO2, CO, and H-2. The decomposition behavior can be reasonably described by a first-order serial reaction. Though 5-HMF has been thought to reduce the carbon gasification efficiency in biomass conversion by initiating the formation of tar and char, the reaction of 5-HMF alone in water did not produce any detectable tar or char under our experimental conditions.