Soil suspensions were maintained at various fixed redox levels to assess the effect of redox on the solubility and availability of Mn. After 4 weeks of incubation, the liquid phase of the suspensions was analyzed, and the activity of Mn2+ was calculated. The results indicated that Mn solubility was controlled by MnCO3 (rhodocrosite) below pe + pH = 16. Literature findings also suggest the presence of MnCO3 when Mn2+ activities were calculated and plotted as a function of redox. When rice plants (Oryza sativa) were grown in the controlled redox suspensions, the uptake of Mn by rice plants was highly correlated with the activity of Mn2+ in solution. The study demonstrates the usefulness of chemical equilibrium relationships to predict the solubility of Mn in the soil and its availability to plants.