The I– adsorption on a modified mineral, which is prepared by impregnating Ag(l) nitrate complex with thiourea(tu) into montmorillonites, have been studied by batch method. The modified mineral was found to adsorb I– ions selectively even under the coexistence of Cl– ions. The distribution ratio(Kd) is 5000 ml/g under the conditions of initial [I–] = 1.0 × 10−5 M(M = mol/dm3), pH = 6.4, and solution/modified mineral = 100 ml/g. It was found that the [Ag(tu)]I precipitates are formed by mixing Nal solution with Ag(l)-tu complex solution, while any precipitates are not formed in addition of NaCl solution, and that anion exchange property of the modified mineral is low. Furthermore, it was confirmed that the [Ag(tu)]I complexes are present in the modified minerals after adsorbing I– ions. From these results, the I– selective adsorptivity of the modified mineral was considered to be due to the formation of [Ag(tu)]I precipitates within the interlayer region of montmorillonite.