D/H and 18O/16O ratios were determined on minerals and whole-rock samples of fresh granites, granites altered during greisenization and kaolinization of the Hercynian Cornubian batholith, detrital kaolinitic sediments of the Bovey Formation, and modern meteoric waters. The granites are high−18O rocks with δ18O = 10.8 to 13.2‰ due to melting, assimilation and/or exchange with argillaceous-rich metasediments at depth. Vein-controlled greisening was dominated by meteoric-hydrothermal fluids, Kaolinites from the major china clay deposits are of weathering origin. They are isotopically consistent with having formed in a tropical to warm temperature climate during the Cretaceous-Tertiary. Although intense supergene kaolinization was probably localized by earlier post-magmatic processes, there is no evidence that china clay kaolinites originally formed during hydrothermal activity and subsequently underwent post-formational isotopic exchange.