Effects of low temperatures upon fine structures of cotton pretreated with either liquid ammonia or caustic of mercerizing strength were investigated. Temperatures of cotton after impregnation with liquid ammonia were lowered by (1) liquid nitrogen, (2) acetone–Dry Ice, (3) acetone, or (4) simple evaporation. With alkali-impregnated fabrics, temperature was lowered by immersion in liquid nitrogen. Changes in fine structure of ammonia- and alkali-celluloses were evaluated by x-ray diffraction before and after removal of each swelling reagent. After treatment with either ammonia or caustic soda, extent of crystalline modification was increased as temperature was reduced. X-ray diffractograms taken while liquid ammonia was still present indicated that the cellulose I structure of cotton sheeting was converted to cellulose III at liquid nitrogen temperatures; extent of conversion was approximately that obtained when a looser yarn structure was treated with liquid ammonia and the ammonia was removed by evaporation. When ammonia was removed with water, the cellulose I lattice was regenerated. In the presence of 23% NaOH, diffractograms indicated a significant decrease in order after a similar drop in temperature, but patterns were not of cellulose II until the NaOH was removed with water. Conversion to cellulose III or to cellulose II was achieved instantaneously when ammonia- and alkali-impregnated fabrics were immersed in liquid nitrogen.