Deuteration of cellulose in the fiber form has been studied by the new technique developed recently. Infrared spectra in the 3-μ region were recorded to obtain four well-resolved bands due to H-bonded hydroxyl groups in the ordered regions. The peaks at 3298 and 3400 cm−1 were assigned to cellulose I, while those at 3445 and 3493 cm−1 were assigned to the cellulose II lattice structure. Cotton fibers containing both cellulose I and cellulose II lattice structures (50:50%) gave six bands in the 3-μ region at 3230, 3298, 3350, 3400, 3445, and 3493 cm−1. Deuteration of swollen and decrystallized cotton fibers revealed that ethylenediamine brings about partial conversion of cellolose I lattice structure to cellulose II lattice structure, giving the characteristic cellulose II band at 3493 cm−1. Swelling mechanism with ethylenediamine to account for formation of cellulose II in cotton fibers has been put forward. Zinc chloride was found to retain the cellulose I structure intact. In both the decrystallized celluloses, the band at 3400 cm−1 has been found to be completely wiped out. This band has been assigned to the group of H bonds mainly lying in rather imperfectly ordered crystalline regions in the fiber structure, which are attacked first during swelling treatment by both the swelling agents irrespective of their different mechanisms of swelling.