IR spectroscopic and X-ray diffraction analysis were used for measuring crystallinity of cotton fibers slack-mercerized in aqueous ammonia/sodium hydroxide solution. The mercerization treatments led to a sharp decrease of crystallinity. Moisture uptake was used for estimating cellulose accessibility. The rate of dye adsorption of slack mercerized cottons increased compared with that of untreated cotton regardless of C.I. Reactive Blue 2 or C.I. Reactive Blue 19. The rate of dye adsorption of ammonia treated cottons increased compared with that of sodium hydroxide treated cottons.