The silica gel surface immobilized with 4-phenylacetophynone 4-aminobenzoylhydrazone (PAAH) after surface modification by 3-chloropropyltrimethoxysilane (CPTS). The modified silica gel was used for sorption of Cu(II), Ni(II) and Co(II) in aqueous solution. The effect of solution pH, sorption time, temperature and initial metal ion concentration onto metal ions sorption was investigated. The characteristics of the sorption process were evaluated by using the Langmuir, Freundlich and Dubinin-Radushkevich (D-R) adsorption isotherms. The sorption of metal ions onto modified silica gel correlated well with the Langmuir type adsorption isotherm and adsorption capacities were found to be 0.012, 0.014 and 0.018 mmol g-1 for Cu(II), Ni(II) and Co(II) metal ions, respectively. From the D-R adsorption isotherms, it was concluded that chemical interactions and chelating effects were playing an important role in the sorption of metal ions onto modified silica gel. Thermodynamic parameters such as the standard free energy change (ΔG)°, enthalpy change (ΔH°) and entrophy change (ΔS°) were calculated to determine the nature of sorption process. From these parameters, ΔH° were found to be endothermic values: 38.39, 18.0, and 14.7 kJ mol-1 for the same sequence of divalent cations and ΔS° values were calculated to be positive for the sorption of each metal ion onto the modified silica gel. Negative ΔG° values indicated that sorption process for all metal ions were spontaneous in nature although they presented an endothermic enthalpy for the interaction, resulting in an entropically favoured process. © 2007 Elsevier B.V. All rights reserved.