The St Austell Granite contains important kaolin deposits, the formation of which is chronologically the last identifiable mineralization episode in the pluton. Field evidence demonstrates that kaolinization is spatially associated with hydrothermal quartz veins and progressed through the migration of alteration fronts away from reactivated pre-existing hydrothermal vein structures. Kaolin veins, sometimes with a quartz core, also occur. Kaolinization results in the formation of a kaolin+smectite assemblage, with no precipitation of quartz. Geochemical models of kaolinization at 25–100 °C show that appropriate mineral assemblages are produced from parent granite by reaction with meteoric water at these temperatures, and with high salinity brine at 75 and 100°C. The aolin+smectite mineral assemblage does not reflect conditions of equilibrium, but reflects elevation of fluid silica activities in excess of quartz saturation as a consequence of feldspar dissolution. Quartz fails to nucleate because of slow precipitation kinetics at the temperatures of kaolinization (<100°C). Kaolinization of the granites was a dynamic process, controlled by feldspar dissolution kinetics, the precipitation kinetics of silica minerals and the initial log a (K+/H+) of the fluids that interacted with the granites.