Abstract The Grahamstown kaolin deposits in the Republic of South Africa occur associated with two distinct geomorphological features, the Grahamstown peneplane (at about 650 m elevation and of Miocene age) and the Coastal plain (at about 520 m elevation and of some later Tertiary age). The parent rocks consist of three lithologically different shales and a diamictite of Devonian to Permian age. The clay occurs in irregularly shaped deposits having no relationship to the structure and folding or to the strati-graphic position within the sedimentary assemblages. The clay deposits are covered by thick silcrete horizons, suggesting that the deposits formed under semi-arid conditions in very flat areas having a fluctuating water table. Pyrophyllite formed prior to kaolinite in those clay deposits that developed from sediments containing both 2 M 1 and 1 M micas as the only silicate minerals. At the beginning of weathering the less stable 1 M mica appears to have favored the formation of pyrophyllite, and only at a later stage of weathering did kaolinite become more stable than pyrophyllite. The recent discovery of alunite, natroalunite, and wardite veins indicates that, beside supergene weathering, some low- to moderate-temperature hydrothermal activity may have contributed to the genesis of the kaolinitic clay deposits.