Silica gel was chemically modified with (3-anilinopropyl) trimethoxysilane (AnPS) under a reflux of toluene (abbreviated as AnPS-SiO2). The adsorptivity of phosphododecatungstate anion (PW12) on AnPS-SiO2 from an aqueous solution was investigated quantitatively, The amounts adsorbed rose quite steeply to a coverage (theta) of 0.90 with increasing PW12 concentrations in solution, while they rose gradually to 0.33 on the unmodified SiO2 as a reference, It was verified, from the changes in bands due to the anilino group in FT-IR spectra and XP-spectra, that such a strong affinity was caused by acid-base interaction of PW12 with the amino group of AnPS-SiO2.