The ability of chitosan as an adsorbent for the removal of acid dyestuff, namely, acid green 25, acid orange 10, acid orange 12, acid red 18, and acid red 73, from aqueous solution has been studied. The experimental equilibrium data for the single component dye-chitosan systems have been analyzed using the linearized forms of Langmuir, Freundlich, and Redlich-Peterson isotherms. The Langmuir isotherm was found to provide the best theoretical correlation of the experimental data for the adsorption of all five acid dyes. On the basis of the Langmuir analysis, the monolayer adsorption (saturation) capacities were determined to be 645.1, 922.9, 973.3, 693.2, and 728.2 mg of dye per gram of chitosan for acid green 25, acid orange 10, acid orange 12, acid red 18, and acid red 73, respectively. The differences in adsorption capacities may be due to the effect of molecular size and the number of sulfonate groups of each dye. The results demonstrated that monovalent and/or smaller dye molecules have superior adsorption capacities due to an increase in the dye/chitosan ratio in the system. The smaller dye molecules are able to undertake a deeper penetration of dye into the internal pore structure of the chitosan particles.