A comparison is made of the mechanochemical activation of three low- and one high-defect kaolinite using a combination of X-ray diffraction, thermal analysis, and DRIFT spectroscopy. The effect of mechanochemical alteration of the kaolinites is greater for the low-defect kaolinites. The effectiveness of the mechanochemical treatment is represented by the slope of the d(001) peakwidth-grinding time line. High-defect kaolinite is not significantly altered by the grinding treatment. The effect of mechanochemical treatment on peakwidth was independent of the presence of quartz; the quartz acts as an additional grinding medium. The effectiveness of the mechanochemical treatment depends on the crystallinity of the kaolinite. Two processes are identified in the mechanochemical activation of the kaolinite: first the delamination of kaolinite appears to take place in the first hour of grinding and second a recombination process results in the reaggregation of the ground crystals. During this process proton hopping occurs and reaction to form water takes place. This water is then adsorbed and coordinated to surface-active sites created during mechanochemical treatment. (C) 2003 Elsevier Inc. All rights reserved.