Changes in the internal morphology of lint cotton fibers during purification and swelling treatments were investigated by measurement of nitrogen adsorption at liquid nitrogen temperature. Surface areas and pore size distributions were calculated from the adsorption isotherms and compared with findings from measurements of changes in crystallinity and in fibrillar aggregation as shown by electron micrographs. The adsorption measurements were made on fibers which were either preswollen in water or taken from the rinse water after treatment and solvent exchanged through methanol and n-pentane to helium. The large changes found in surface area, pore size distribution, and fibrillar aggregation were accompanied by changes in crystal structure in fibers treated with 5.0M sodium hydroxide. Large changes following treatment with phosphoric acid occurred in pore structure and fibrillar aggregation without concomitant change in crystallinity.