Montmorillonite materials before and after intercalation of OH-Cr(III) species were analyzed thermically up to 1000degreesC in air and nitrogen atmospheres. The thermal treatments were obtained by using thermal analysis equipment and the structural changes were followed by X-ray diffraction. Compared to the original montmorillonite, the montmorillonite treated with OH-Cr species showed differences in both thermal processes. The 450degreesC thermal treatment on OH-Cr-montmorillonite in air and N-2 produced different interlayer spacing. The 1000degreesC thermal treatment, where clay has already collapsed, also present differences in the obtained products: eskolaite (Cr2O3) in air and Mg-Cr-spinel in nitrogen. (C) 2002 Elsevier Science B.V All rights reserved.