The adsorption behavior of divalent cations M2+ (Cu, Ni, Co and Zn) with commercial humic acid (HAA1) and also with an extracted fraction of peat soil (HAPs) was followed in aqueous solution. The series of adsorption isotherms were fitted to a modified Langmuir equation. The maximum number of moles adsorbed gave: 0.55 +/- 0.02, 0.66 +/- 0.02, 0.54 +/- 0.02, 0.40 +/- 0.02 mmol per gram for HAA1 and 0.63 +/- 0.03, 0.61 +/- 0.06, 0.55 +/- 0.02, 0.54 +/- 0.03 mmol/g for solid HAPs, for copper, nickel, zinc and cobalt, respectively. The same interaction followed calorimetrically gave endothermic values: 2.4 +/- 1.0, 8.4 +/- 0.9, 18.3 +/- 0.9, 10.6 +/- 0.9 kJ mol(-1) and 18.4 +/- 1.2, 15.9 +/- 1.4, 15.4 +/- 1.2, 15.0 +/- 1.2 kJ mol(-1) for HAA1 and HAPs, respectively, for the same sequence. Because all Gibbs free energies were negative. Complexation must be accompanied by an increase in entropy. (C) 2003 Elsevier B.V. All rights reserved.