The study investigates the effect of adding oxygen pressure and hydrogen peroxide to the soda pulping process on the crystalline structure of cellulose in cotton linters, by X-ray diffraction and FT-IR spectroscopy methods. Increase of cellulose's crystallinity in pulp was determined when oxygen pressure and peroxide were added to soda pulping. The disorder parameter of cellulose in the pulp samples obtained from soda-oxygen and soda-oxygen-peroxide cooking at 105 degrees C obviously increased. The crystallite size of cellulose (002 lattice plane) in the pulp samples changed considerably between 5.4 and 5.9 nm. On the other hand, the unit cell type of cellulose in cotton linters was not affected by the chemical treatment in cooking trials. The monoclinic structure was dominant in soda, soda-oxygen and soda-oxygen-peroxide pulp samples.