The interaction of the antifolate drug pyrimethamine with chitosan derivatives, namely, chit-GLT (glutaraldehyde cross-linked chitosan gel beads) and chit-Cu (chit-GLT beads treated with copper solution) has been investigated from isothermal calorimetry at 298 and 308 K, being studied in terms of the Avrarmi and pseudo-first- and second-order kinetic models. The Avrami model provided a high fitting with the experimental data. The results have pointed out the slower kinetic for pyrimethamine sorption on chit-GLT compared to chit-Cu, in agreement with the reaction time obtained from calorimetry. (c) 2007 Elsevier Ltd. All rights reserved.