Physico-chemical studies on the nature of adsorbed cations held on kaolinite surface, i.e. the change of the mean free bonding energy of adsorbed H+, and of the exchangeabilities of Li+, Na+, K+, Cs+ caused by drying the kaolinite below 600°C, were carried out.It was concluded that kaolinites, regardless of their morphological properties, have power to fix almost nonexchangeably a considerable amount of above cations. The ability of fixing the cations increases with the increasing degree of drying, until kaolinite decomposes with the loss of lattice water, when the decrease of fixing power is observed. The fixing power varies a great deal with the radii of cations as well as with the degree of losing the water of hydration at a definite temperature, the samller cation and the larger degree of dehydration causing the more powerful fixation.Drying brings with the decrease of ζ-potential of the clay surface suspended in water, and therefore the rehydration of adsorbed ions may not be reversible at normal temperature.A new theory of the fixation of adsorbed cations, being quite different from any current theories which have been introduced so far by soil scientists, is advanced by authors. As the fixation causes the change of ionic distribution in electrical diffuse double layer, it would be natural to consider that the fixation may affect the properties of kaolinite-water systems. The influences of drying on the sedimentation of clay-water suspension were investigated. It was noted that the kaolinite dried even at such a low temperature as below 50°C showed a remarkable differences in sedimentation volume and settling rate from those of non-dried, original suspension. The authors are going to point out that the fixation phenomenon, first unveiled by them, would be very significant for the colloid chemical or rheological study of clay minerals and for the proper control in clay industries.