Gypsum is an useful soil amendment. However, application of gypsum sometimes enhances the dispersion of highly weathered acid soils. The mechanism of the effect of gypsum on the dispersion of a Japanese acid soil is discussed in terms of adsorbed ions and aggregate stability. Leaching of the soil column by NaCl,Na2SO4, CaCl2, and CaSO4 electrolyte solutions showed that Ca2+ could displace Al3+ in the soil, and SO2- caused more cation retention of the soil. In the slaking experiment, the soil was most dispersive with the CaSO4, while it was least dispersive with the NaCl. The result goes against the effect that gypsum has on sodic soils. It was concluded that removal of Al3+, as a binding agent of the acid soil, by Ca2+ and the change in charge characteristic due to SO42 adsorption were the mechanisms that enhanced dispersion of the Japanese acid soil following gypsum application.