The preparation of the fibrous ion exchangers containing cellulose as reinforcement is studied. Monomer mixtures, such as methacrylic acid-divinyl benzene, are impregnated in di- or triacetate fibers, and polymerized in-fiber. After hydrolysis of the cellulose acetate fibers, though in some cases the hydrolysis is not necessary, ion exchange groups are introduced. The results obtained are summarized as follows:1) Each type of fibrous ion exchanger, i.e. strong acidic cation exchanger, weak acidic cation exchanger, strong basic anion exchanger, and weak basic anion exchanger, is obtained in good fibrous conformation.2) The ion exchange capacities measured agree well with the values calculated on the quantity of the polymer introduced in the original fibers.3) The water swelling property is controllable by changing the degree of crosslinking (DVB%), but for the low degree of crosslinking, it is also influenced by the quantity of the polymer introduced.