A series of works carried out in the Laboratory of Soil Electro-chemistry of this Institute during the last ten years showed that the quantity factor (amount of the reducing compounds) and the intensity factor (redox potential) are of equal importance in characterizing the oxidation-reduction status of paddy soils in relation to its influences on soil properties and rice growth. The reducing compounds were estimated by potentiometric titration, chemical determination, and polarographic characterization. The redox potential of a given soil is determined primarily by the nature and amount of the organic reducing compounds, which are rather complex in composition and may range in half-wave potential from about 0.22V to 0.71V vs. calomel electrode under water-logged conditions. Owing to the reduction of ferric oxides by the organic reducing compounds, however, ferrous iron appears gradually and at the stage of transformation equilibrium ferrous iron may amount to 70-90% of the total reducing compounds. The ferrous iron exists in forms of free ions in solution, water-soluble chelates, exchangeable ions, chelates associated with the solid phase of the soil, and precipitates as sulfide and hydroxide.