The thermal properties and structures of deoxyhalocelluloses were studied using thermogravimetric and differential scanning calorimetric analyses and an X-ray diffraction method. The X-ray diffraction curves of the cellulose derivatives were obtained after subjecting them to a specific treatment that was supposed to render a specific cellulose allomorph from the amorphous state cellulose; all of the cellulose derivatives examined without such treatment exhibited no crystalline features. Only deoxyfluorocellulose was found to give a diffraction pattern similar to that of the corresponding cellulose allomorph, which was cellulose IV in this case. On the basis of the thermal analyses, it was confirmed that chlorodeoxycellulose has a flameretardant property, whereas deoxyfluorocellulose was found not to be flame-retardant. The difference is supposedly due to the difference in the bonding energy between C-Cl and C-F.