Swelling and decrystallizing action of aqueous solutions of benzylamine and mixtures of 4-amino acetophenone with methanol was studied on cotton fibers. The extent and type of swelling were studied by determining various characteristics of the swollen cotton, viz., moisture regain, acetyl number, infrared crystallinity index, and water of imbibition values. Benzylamine was found to be an intracrystalline swelling agent, while 4-amino acetophenone brought about changes in the accessible regions only. Benzylamine has been regarded as substituted methylamine, and its swelling and decrystallizing action on cotton fibers has been explained on the basis of its ability to form specific hydrates with water. The intercrystalline swelling action of 4-amino acetophenone resulted in further opening up of the accessible portions of the fiber. This was explained on the basis of its inability to form any specific complexes with methanol as well as its low basicity. The weak basic character of 4-amino acetophenone was attributed to the substitution of deactivating and meta-directing acetyl group in the para-position to the amino group of aniline.