A new type of adsorbent based on the immobilization of humic acid (HA) extracted from tropical peat soil taken in Sambutan village, East Kalimantan, Indonesia, on silica gel (SG) was synthesized and then applied as an adsorbent for the selective adsorption of Cu(II) in the presence of Ca(II). The extraction of HA was performed by the recommended procedure of International Humic Substances Society (IHSS). The immobilization was begun with purification of SG in HCl 1:1 and followed by activation at temperature 110°C for 24 hours. The activated silica gel (4 g) was suspended into 40 mL of NaOH 0.01 M, and into this suspension, 60 mL of NaOH 0.01 M containing 4 g HA was then poured and stirred for 24 hours to allow immobilization of HA on SG. This immobilization produced an adsorbent (SG-HA) with the content of HA was 13.05%(w/w), and this immobilized HA was stable in the acidity range of pH 1.0 to 8.0. The application of SG-HA to selectively adsorb Cu(II) from the solution containing Ca(II) could be achieved at medium acidity that equivalent to pH 4.0 to 5.0. At pH 5, the adsorption rate constant of Cu(II) on SG-HA was 0.027 min^-1, while that of Ca(II) on SG-HA was only 0.004 min^-1. Similar to the adsorption rate constant, the adsorption capacity and energy of SG-HA for Cu(II), i.e. 0.21 mmol/g and 26.63 kJ/mol, respectively, were also higher than those for Ca(II), i.e. 0.07 mmol/g and 24.87 kJ/mol, respectively. Compared to SG, the immobilization of HA on SG resulted enhancement of the rate constant, capacity, and energy of adsorption. By using SG, the rate constant, capacity, and energy of adsorption of Cu(II) were 0.001 min^-1, 0.10 mmol/g, and 21.34 kJ/mol, respectively. [DOI: 10.1380/ejssnt.2006.602]