The stability constants of Fe(Ⅱ)-complexes were determined on the basis of the shift in peak potential during the reduction of Fe²⁺ in the presence of eomplexants. The log K values for soil solutions ranged from 2.6 to 4.5. A chang of pH by 1 unit induced a change of log K by 0.92 unit on average. Aeration of the soil solution led to an increase in logK value. The logKvalues for Fe(Ⅱ)-complexes were larger than those for Mn(Ⅱ)-complexes by about 0.8 unit.