Eighteen purified kaolin samples from Thai Ultisols were studied by X-ray diffraction, X-ray fluorescence, transmission electron microscopy and BET methods. Minor amounts of inhibited vermiculite, quartz and anatase were general contaminants of the kaolins which had an average chemical composition of 403 g kg(-1) Al2O3, 550 g kg(-1) SiO2, 25.3 g kg(-1) Fe2O3, 15.6 g kg(-1) TiO2 and 4.65 g kg(-1) K2O on an ignited basis. Appreciable concentrations of Mn, Co, Ni, Cu, Zn, As and Pb were present and most of the Ni, Cu and Zn in the original clay fraction was retained in the deferrated kaolin concentrate. It was not possible to determine if these elements are present as structural ions in kaolin crystals.The kaolins exhibited a variety of crystal morphologies ranging from sub-micron, euhedral, hexagonal plates to anhedral plates and tubes. Their specific surface areas ranged from 15.9 to 61.4 m(2)g(-1) (mean 44.9 m(2)g(-1)) and surface area increased with decrease in crystal size. The cation exchange capacity of the kaolins ranged from 7.2 to 23.4 cmol(c) kg(-1) and surface charge density from 0.16 to 0.99 C m(-2) but these values are sensitive to the presence of contaminants. Structural iron ranged from 12.4 to 44.8 g kg(-1) Fe2O3 and there was an increase in structural defects towards the soil surface associated with an increase in the amount of structural iron.