Isothermal iodine adsorption behaviors were studied with seven natural and regenerated cellulose fibers. The equilibrium iodine adsorption was determined by the quick adsorption of iodine from concentrated iodine. KI solution followed by the slow desorption with conc. Na$_2$SO$_4$ solution, originally developed by Schwertassek and modified by Nelson et al. The interpretations of the adsorption data by use of Langmuir and BET equations agreed to each other, indicating that iodine was adsorbed to form a monolayer on the cellulose innersurface.