The present study dealt with the adsorption of eosin Y, as a model anionic dye, from aqueous solution using chitosan nanoparticles prepared by the ionic gelation between chitosan and tripolyphosphate. The nanoparticles were characterized by atomic force microscopy (AFM), size and zeta potential analysis. A batch system was applied to study the adsorption of eosin Y from aqueous solution by chitosan nanoparticles. The results showed that the adsorption of eosin Y on chitosan nanoparticles was affected by contact time, eosin Y concentration, pH and temperature. Experimental data followed Langmuir isotherm model and the adsorption capacity was found to be 3.333 g/g. The adsorption process was endothermic in nature with an enthalpy change (Delta H) of 16.7 kJ/mol at 20-50 degrees C. The optimum pH value for eosin Y removal was found to be 2-6. The dye was desorbed from the chitosan nanoparticles by increasing the pH of the solution. (c) 2007 Published by Elsevier B.V.