In this study, the rate of xylan hydrolysis was investigated. The hydrolysis of tulip poplar was conducted in subcritical water from 180 to 220 degrees C for 1-240 min. An equation for estimating the hydrolysis of ether bonds in both xylan and xylooligosaccharide from the yield of xylan and xylooligosaccharide was derived, and the rate constant of ether bond hydrolysis was calculated using the derived equation. It was found that our kinetic study can be applied to subcritical water treatment of various lignocellulosic biomass. In addition, linearity between the apparent activation energy and the logarithmic value of the apparent pre-exponential factor was observed. An equation for the apparent reaction rate constant was derived by introducing a factor, a, that reflects the changes in the reaction rate arising from various factors.