Abstract: Theory of electric double layer shows that the cationic type of aqueous solution and its concentration influence dispersivity of clayey soil. The dispersive characteristics of this soil in a soil-water-electrolyte system were studied for solutions containing different concentrations and cations through pin-hole tests, crumb tests, and disintegration tests on non-dispersive soil, transitional soil, and dispersive soil. The results show that non-dispersive soil behaves dispersive or transitional in a solution containing a certain content of alkaline and sodium ions and conversely, the dispersivity of both dispersive soil and transitional soils is weakened gradually by an increase in the concentration of electrolyte solution. For dispersive soil in deionized water, if its dispersivity is increased, the speed of its disintegration becomes slower and a longer time is needed for a full disintegration. For non-dispersive soil in any types of electrolyte solution or in deionized water, the rate of its disintegration is nearly the same, while dispersive soil in a solution of greater electrolyte concentration takes a faster rate and a shorter time of full disintegration, gradually approaching those of non-dispersive soil. A most prominent behavior of dispersive soil in disintegration is a mutation point that appears in a disintegrated soil-weight range of 20%~40% as a threshold beyond which a full disintegration will be completed in a very short time. The results also indicate that clay soil varies in dispersivity with its external media environment. Thus, a change in dispersivity of clay can be achieved in practical engineering through a change in the concentration of local seepage water. This study suggests that great attention should be paid to the mutation of dispersive soil in its disintegration, so as to avoid a rapid destruction of hydraulic structure beyond the threshold.