The shifts of the Si-O-Si stretching and the Si-O-Al and Si-O-Si bending modes, as well as the free silica peak in the IR spectra and the strengthening of the XRD reflections due to the quartz and alpha-cristobalite components of the Al-pillared bentonites (Wyoming and Resadiye), are ascribed to the formation of new Si-O-Al groups of covalent character. The mass losses in the temperature range 150-700 degrees C correspond to the dehydration and dehydroxylation processes. The total mass losses of the Al-pillared bentonites (Wyoming and Resadiye) are close to each other at an OH/Al ratio of 2.2, but calcination from 400 to 600 degrees C causes the surface area of the latter composite to decrease by 13%. (c) 2007 Elsevier Inc. All rights reserved.