Oxidation-reduction reactions are chemical reactions caused by the transfer of electrons between two substances. These reactions occur actively in variable charge soils. This is because that under conditions of high temperature and high precipitation both the accumulation and the decomposition of organic matter proceed rapidly. The decomposition products of organic matter may release electrons, providing the necessary condition for the occurrence of reduction reactions. In particular, because the soil may have a high content of water during seasonal rainy periods, the presence of a strongly reducing condition is possible. Furthermore, large areas of variable charge soils have been cultivated for rice production. For these paddy soils there are always intensive oxidation-reduction reactions proceeding alternately. Variable charge soils have a high content of iron oxides. The content of manganese is also higher than that of constant charge soils. Thus, the soil itself possesses plenty of electron-acceptors. Besides, the high concentration of hydrogen ions in variable charge soils is favorable for the occurrence of reduction reactions. Therefore, as shall be seen in this chapter, contrary to the belief that the significance of oxidation- reduction reactions is confined chiefly to submerged soils, these reactions may play an important role in soil genesis and soil fertility for variable charge soils even under well-aerated conditions. In this chapter, after discussions on factors affecting the intensity of oxidation-reduction and interactions among various oxidation-reduction substances, the oxidation-reduction regimes of variable charge soils under different utilization conditions will be presented. Ferrous and manganous ions, two important inorganic reducing substances in soils, shall be dealt with in the next chapter. The oxidation-reduction intensity of a substance is determined by its ability to liberate or accept electrons. Therefore, electron activity in an equilibrium system may be used as an index for expressing its reduction strength. An electron has a radius of only approximately 1/20,000 of that of a hydrogen atom. Its large charge-to-size ratio prevents it from persisting in free form in aqueous systems. The ephemeral “hydrated electron” has a half-life of less than 1 msec (Bartlett and James, 1993). As a species with a potential of -2.7 V vs. the standard potential of H+/H2, it is a powerful reducing agent.