This paper presents preliminary quantitative results on the chemical and structural alterations of modified montmorillonite samples. The chemical composition analysis of the acid activated samples reveals that the decolouring capacity of the activated product reaches its maximum value of 210 when the total dissolving rate of the octahedral cations reaches about 30%. And, along with the dissolving of the cations the crystalline degree of the activated montmorillonite represented by the b-axis order degree of montmorillonite decreases to some extent. Structural alterations were also observed on the Na-modified and the organically modified samples. The d((001)) of the Na-modified sample changes to 12.65 Angstrom from 15.49 Angstrom of the natural sample, and the d((001)) of the organically modified sample expands to 16.54 Angstrom from 12.65 Angstrom when treated with tetramethylammonium and to 23.11 Angstrom when treated with octadecyltrimethylammonium. (C) 2003 Elsevier Ltd. All rights reserved.