Chitosan, a deacetylated polymer of chitin, was applied and characterized as an adsorbent for a dye Toluidine Blue O. The adsorption of Toluidine Blue O by chitosan was affected by the particle size and mass of chitosan, initial dye concentration, reaction time, and solution pH. More dye was adsorbed the smaller the chitosan particles. When initial ratio of dye to chitosan was over 1 : 500, the adsorption of dye was rapidly declined. Adsorption rate of dye to chitosan showed 2 phases, rapidly occurred one at the beginning within 3 min and thereafter very slowly occurred one. The amount of dye adsorbed was increased with increase in pH.