Copper ion-exchanged montmorillonite clays, which had been prepared by ion exchange in an aqueous solution Of CuCl2 at a temperature above 323 K, exhibited the characteristic IR band in the region 3360-3310 cm(-1). No such bands were observed for the samples prepared by using different ion-exchange solutions and at different temperatures. From the spectroscopic observations, it was revealed that the ion-exchanged copper ions (Cu2+) are in the form of dimerized species by bridging two hydroxyls, [Cu2+ < (OH-)(2) > Cu2+], in montmorillonite. (c) 2006 Elsevier Inc. All rights reserved.