The adsorption of azobenzene FD&C Red No. 40 (C.I. 16035) from aqueous solutions by chitosan was studied through adsorption isotherms. The effects of pH (5.7, 6.6 and 7.5). particle size ranges (0.10 +/- 0.02, 0.18 +/- 0.02 and 0.26 +/- 0.02 mm), deacetylation degree (42 +/- 5%, 64 +/- 3% and 84 3%) and temperature (25, 35 and 45 degrees C) were investigated. Langmuir, Freundlich and Redlich-Peterson (R-P) adsorption models were applied in order to describe the experimental isotherms and isotherm constants. Coefficients of determination (R(2)>0.95) and mean relative error (MRE<0.10) values showed that Langmuir and R-P models presented better fit with the experimental data. The maximum monolayer adsorption value has been found to be 529 mg g(-1), at pH 6.6, temperature 35 degrees C. particle size range 0.10 +/- 0.02 mm, and deacetylation degree 84 3%. (C) 2009 Published by Elsevier Ltd.