The exfoliation characteristics of Karakog phlogopite (KP), Phalaborwa phlogopite (PP) and Phalaborwa vermiculite (PV) with hydrogen peroxide were investigated. A series of swelling tests were done to obtain comparative data in order to determine the effects of hydrogen peroxide concentration (1-50 wt.%) and reaction time (1-30 h) that were determined to be the main parameters with solid/liquid ratio being of minor importance. Experimental results show that Karakog sample has higher exfoliation rate and Phalaborwa samples show a steep increase in exfoliation rate when hydrogen peroxide concentration exceeds 30%. Maximum swelling values of 103-, 90- and 118-fold were reached at 50% H2O2 concentration in 30 h for KP, PP and PV samples, respectively. Exfoliation mechanism was explained in terms of crystal water contents, released cations and change of pH values of solutions. (C) 2002 Elsevier Science Ltd. All rights reserved.