Pure nano-montmorillonite (MMT) with interlayer cations of tetramethylammonium (TMA(+)) was prepared by the two different procedures in supercritical water (SCW). In single-step procedure, synthesis of a 1-month aged precursor was conducted directly in SCW medium; while, in two-step procedure, a temperature programmed-promotion approach was applied. For the later technique, first the aging of the fresh solution of reactants was carried out for 2 h at 80 degrees C followed by temperature increment to reach supercritical condition (T>372 degrees C, P > 221 atm). The prepared powders were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The Al/Si ratio of synthesized montmorillonite was about 1.73 as determined by inductively coupled plasma optical emission spectrometry (ICP-OES). Furthermore, the effects of operational parameters, i.e. temperature, reaction time, and precursor aging were studied on purity and crystallinity of product. In spite of the high crystalline montmorillonite obtained from the two procedures, it was revealed that the two-step procedure had a significant effect on the crystallinity, surface area, and consequently, adsorption capacity of products compared to MMT obtained from a single-step method. High adsorption capacity indicates that crystalline TMA-montmorillonite can be synthesized efficiently using two-stage procedure in supercritical water. Possible mechanism of montmorillonite synthesis in supercritical water was also suggested. (C) 2014 Elsevier B.V. All rights reserved.