Benzo-crown ether-functionalized silica (BCES) was prepared by the combination of the sol-gel process and the Schiff base reaction, characterized by Fourier transmission infrared spectroscopy, scanning electron microscopy, and N-2 adsorption-desorption, and used for the selective adsorption of Ca2+ ions. The adsorption amounts of the BCES for Ca2+ ions were independent of pH in the range of 5-10. Adsorption equilibrium was performed within 60 min. The adsorption capacity of the BCES for Ca2+ ions was found to be 40.6 mg g(-1) at 298 K. The high selectivity of the BCES for Ca2+ ions with respect to the other alkali and alkaline earth metals was attributed to the high affinity between Ca2+ ions and crown ether. The adsorption/desorption cycle of the BCES could be up to five times. Langmuir and pseudo-second-order equations provided good correlation for the adsorption of Ca2+ ions. The thermodynamics of Ca2+ ions/BCES system indicates spontaneous and endothermic nature.