A thermodynamic investigation of the interaction of pyrimethamine (PYR) and sulfadiazine (SDZ) with immobilized copper on chitosan (chit-Cu) was done by heat-conduction calorimetry at 298.15 K. Langmuir isotherm describes the adsorption equilibrium behaviour in the entire concentration range studied. The molar enthalpies for formation of a monolayer of drug, Delta H-mon(m), from linear and (non-linear) data analysis methods were found to be -40.2 +/- 1.2 (-42.1 +/- 1.5) and -69.0 +/- 2.2 (-68.5 +/- 1.9) kJ mol(-1) for PYR and SDZ, respectively. (c) 2007 Elsevier B.V. All rights reserved.