Dissolving pulp was treated with ammonia gas at 20 °C at different pressure levels. The modified pulp was analysed with regard to the change in crystal structure from cellulose I to III. X-ray diffraction and Raman spectroscopy were used as analytical methods. It was found that the transformation of cellulose from cellulose I to III takes place above a defined pressure level, which could be recorded continuously as a function of pressure. Below the characteristic pressure level, the cellulose is still present as cellulose I after treatment. This pressure level is lowered by the presence of water. The ammonia gas is therefore able to swell the crystalline areas of the cellulose starting from a defined process intensity, causing the cellulose to recrystallize in a different structure. Finally, the cellulose III obtained was partially converted back into cellulose I by water boiling.</jats:p>