H-2 generation during mechanochemical treatment of kaolinite by dry grinding was examined by X-ray diffraction analysis, Fourier transform infrared spectroscopy, and BET surface area measurement. The H-2 concentration in the mill pot, measured by gas chromatography, increased with grinding time up to a maximum concentration of 156 ppm (0.35 mumol) after 600 min. This H-2 generation is considered to occur as a result of three processes: (1) structural destruction characterized by the delamination and loss of hydroxyl groups as a result of dry grinding, (2) transformation of liberated hydroxyls into water molecules by mechanochemical effects such as prototropy, and (3) H-2 generation through reaction between surface water molecules and mechanoradicals created by the rupture of Si-O or Al-O-Si bonds. Although the surface area plateaued after 240 min grinding, the H-2 concentration continued to increase, indicating that surface mechanoradicals are created during this later grinding stage. Thus, H-2 generation can be used as an indicator of mechanoradical formation during mechanochemical treatment. (C) 2004 Elsevier Inc. All rights reserved.