Chitin and chitosan are good removers of cations from aqueous solution and wastewater. The interactive effect of cation with both biopolymers in aqueous medium was studied by the batch method at 298 +/- 1 K. The results were fitted to the modified Langmuir equation. The same adsorption was followed by calorimetric titration. In this process, 50.0 mg of each polymer was suspended in 19.0 cm(3) of bidistilled water at 298.15 +/- 0.02 K, maintained under mechanical turbine stirring. The titration was performed by adding increments of 10 mu L of 0.10 mol dm(-3) Cu(NO3)(2) aqueous solution to the system. The resulting isotherm was also adjusted to a modified Langmuir equation. From the thermal effects K and Delta H values were determined, enabling the calculation of Delta G and Delta S for the interaction of copper cations with chitin and chitosan, giving the enthalpic values of -19.85 +/- 0.34 and -41.27 +/- 1.57 kJ mol(-1), respectively. The spontaneity of this interaction is shown from Delta G values of -35.9 +/- 0.1 and -36.8 +/- 0.1 kJ mol(-1), which are followed by Delta S values of +54 and of -15 J mol(-1) K-1, respectively. The complexation is probably associated with the lack of order of the chitin polymeric chain or with the freedom of water molecules initially bonded to cations. The copper ion is coordinated to the pendant groups of the polymeric chain to form stable complexes.