Summary Decomposition of cellulose crystallites in wood during pyrolysis was studied by X-ray diffraction using a tension wood of Populus maximowiczii (cottonwood), which contains highly crystalline cellulose. X-ray diffraction profiles were recorded at varied temperature up to 360°C. By one-hour isothermal treatments, the cellulose crystallites did not decompose at 300°C, but completely decomposed at 340°C. The change in equatorial diffraction profile was studied by temperature scan up to 360°C and by isothermal treatment at the critical temperature of 320°C. Along with the changes by thermal expansion, the changes in diffraction diagram revealed a characteristic discrepancy between the diminishment of crystalline order and the reduction in crystallite size; i.e., the intensity of crystalline reflections diminished steadily while the crystallite size decreased much more slowly. A model of highly heterogeneous decomposition is proposed to explain this behavior.