Flocculation and dispersion of clay particles in in vitro waterlogged soils at 8, 18, 28, and 380C were investigated with some indications of soil reduction. Above 280C, waterlogging for l-week in the six soils caused a significant decrease in the turbidity of their colloidal suspensions. The decrease in the turbidity, i.e., the flocculation of the clay particles, was evident with (i) a decrease of redox potential (Eh) of the soils, (ii) an increase in the values of electric conductivity (EC) and water-soluble cations (WSCs: Fe2+, Ca2+, Mg2+, NH4+ etc.) in the solutions, and (iii) an increase in the amounts of volatile fatty acids (VFAs: acetic, propionic, and n,-butyric acids) in the soils. Soilincutoation at 8℃ retarded the decrease in Eh, the increase in the values of EC, WSCs, and VFAs, and the decrease in the turbidity. At 380C, samples of four soils water-logged for 4 weeks showed a low turbidity with a low Eh, and high values of EC, WSCs, and VFAs. The samples of other two soils, however, exhibited a significant increase in the turbidity, viz. the dispersion of clay particles, with an increase in pH and decrease in the values of EC, WSCs, and VFAs under low Eh. The observed flocculation and dispersion of clay particles in the water-logged soils were discussed in relation to chemical changes under sequential reduction of soils, particularly change in ionic strength of soil solutions largely due to the (i) dissolution and precipitation reactions related to Fe2+ and Fe(III) and (ii) cation exchange reactions between Fe2+ and other exchangeable cations on clay surfaces.