Surfactant modified montmorillonitic clays synthesized by ion exchange using the hydrothermal reaction method have been compared using XRD and thermal analysis. X-ray diffraction (XRD) shows the changes in the surface properties of organoclays through expansion with surfactant loading. A polynomial relationship exists between the basal spacing and the CEC loading described by the equation y = 0.3232x(2) + 0.2052x + 1.2834 with R-2 = 0.9955. Different arrangements of the surfactant molecules in the organoclays are inferred from the changes in basal spacings. para-Nitrophenol also causes the expansion of the montmorillonite clay and affects the arrangements of the surfactant molecules within the clay layers. Changes in the surfactant molecular arrangements were analyzed by thermogravimetry. Additional thermal decomposition steps were observed when para-nitrophenol is adsorbed on the organoclay. (c) 2006 Elsevier Inc. All rights reserved.