In the present study, the ferrous iron in paddy soils was classified and determined separately into four forms, namely, water-soluble, exchangeable, complexed and precipitated. Laboratory experiments were also made to elucidate their interrelationships with regard to amount, transformation and factors affecting their equilibrium. It was found that for ordinary paddy soils the water-soluble and exchangeable forms ranged each to less than 1-1.5 per cent of the total ferrous iron, and the percentage was each raised to more than 20 after the addition of easily-decomposable organic matter. The amount of ferrous iron complexed by solid phase fluctuated within 16-36 percent of the total ferrous iron. The amount of the exchangeable ferrous iron is influenced by the ration-exchange capacity of the soil, the amount of sulfide present and the prevailing pH. The amount increased with the increasing of soil acidity until pH 4 but decreased thereafter, presumably due to the competition for the exchange site by hydrogen and aluminum ions in the latter case. The adsorption of ferrous iron by soils leads to an equivalent liberation of calcium ions to the liquid phase.