The equilibrium solubility of potassium from the lattice of illites in four soil clays has been measured. This solubility is related to the availability of this potassium and, when account is taken of the fineness of the clay particles, it is found to decrease as the illite structure becomes more degraded by loss of lattice potassium. The solubility depends on the pH of the system in the same way for all the clays, and a minimum value is found near to pH 11. Above this pH, potassium release is due to breakdown of the clay mineral. At lower pH values the release is due either to the instability of the clay mineral structure towards hydrogen ions or to displacement of potassium by other cations acting together with hydrogen ions.