Organoclay was prepared by a precipitation-adsorption process in which sodium-montmorillonite (Na-MMT) suspension was fully mixed with an anionic surfactant, sodium dodecylbenzene sulfonate, and then precipitation agent CaCl2 was added to produce a precipitate of organophilic montmorillonite (OMMT). The Na-MMT suspension was obtained by ion-exchange reaction of bentonite with sodium cation resin. Fourier transform infrared spectroscopy spectral analysis confirmed the adsorption of organic groups on the MMT, and the organic groups content was calculated from the results of thermal gravimetric analysis. X-ray diffraction results indicated that the OMMT prepared was exfoliated. In this article, the mechanism of the precipitation-adsorption process was discussed. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 971-974, 2009