Electron-microscopical techniques were used to illustrate the reduction in the size of the cellulose fibril brought about by treating cotton with aqueous 14% sodium hydroxide and liquid ammonia. Reduction in diameter of fibrils increased internal surface area, which might in turn explain the enhanced chemical reactivity of cotton fibers mercerized with liquid ammonia. cotton fibers were stained positively with an acrylic-osmium complex and negatively with phosphotungstic acid.