The swelling of cotton increased continuously as the concentration of LiOH solution was raised progressively to saturation. In contrast, the swelling of cotton treated with NaOH or KOH leveled off at higher concentrations after increasing initially. The maximum swelling achieved with NaOH was higher than that obtained with LiOH, which was higher than that given by KOH. Accessibility of cotton treated with LiOH solutions, as indicated by sorption ratios, was lower than that of cotton treated with NaOH or KOH solutions of similar concentrations. The level-off degree of polymerization (LODP) of cotton treated with similar concentrations of alkali decreased in the order LiOH, NaOH, KOH. The reactivity, as indicated by acetylation, of cotton treated with alkali metal hydroxides, washed with water, and never dried was measured. At concentrations between 5.5.N and 8.2N, the rate of increase of reactivity was relatively low for cotton treated with NaOH, and no increase was observed for cotton treated with KOH. The reactivity of LiOH-treated cotton continued to increase sharply up to treatment with a saturated solution of the alkali. A limited number of observations were also made on cotton treated with CsOH and RbOH. Accessibility, reactivity, extent of swelling, and LODP values, together with x-ray data, are discussed in relation to the fine structure of the alkali-treated fiber.