Chitosan can use its protonated amine groups to adsorb strongly anionic species from diluted solutions. In this work, adsorption and thermochemical data on the interaction of the dye indigo carmine with chitosan in aqueous medium were found, in order to obtain new adsorption data on this relatively unexplored chitosan field. The studies were carried out by the batch method from 35 to 50degreesC. The adsorption results were well fitted to both Langmuir and Freundlich adsorption models. The increase in the temperature decreased the adsorption of the dye. The enthalpy of interaction, when a monolayer of the dye was formed on the chitosan surface, Delta(int)H, of -23.2 kJ mol(-l) was encountered for all temperature ranges studied. The spontaneity of the interaction is indicated by the Delta(int)G values from -9.1 to -8.2 kJ mol(-1). Other thermodynamic quantities were also calculated and are discussed.