Acetylacetone was covalently immobilized on silica gel to produce a new surface (Sil-ac), having 0.62 mmol of ligand attached per gram of silica, which was characterized by means of elemental analysis and physical measurements, before being submitted to calorimetric measurements, Thus, a suspension of this anchored surface in ethanol or acetone was calorimetrically titrated with divalent cations (Co, Ni, Cu, Zn, Cd, Hg). The resulting isotherms conformed to a modified Langmuir equation. After linearization of these isotherms, the enthalpies of cation/ surface adsorption and the interfacial equilibrium constant, as well as the variation of free energies and entropies, were calculated. In acetone, adsorption of copper gives rise to an exothermic Delta H of -93.38 +/- 0.94 kJ mol(-1), while cobalt, 41.80 +/- 0.42 kJ mol(-1), and zinc, 2.38 +/- 0.02 kJ mol(-1) give an endothermic adsorption. In ethanol, the endothermic Delta H of 52.64 +/- 0.53 kJ mol(-1) for zinc contrasts with the exothermic enthalpies of adsorption observed for other cations, especially for mercury, -102.20 +/- 1.02 kJ mol(-1) and nickel, -137.70 +/- 1.38 kJ mol(-1).