The solubility of potassium ferrate(VI) (K(2)FeO(4)) was measured in a mixture solution containing various molar ratios of KOH and NaOH at a total concentration of 14 M from (10 to 60) degrees C. The solubility of K(2)FeO(4) decreased necause of the presence of K(+) in the solution mixture. The solubility (S*) was expressed as log S* = 5.830 - 2019/T - 5.845X(K+)(0.5) + 1.595X(K+) + 724X(K+)(0.5)/T (sigma = 0.024), where X(K+) is the mole fraction of K(+) in the solution mixture and T is in Kelvin. A comparison of the values of S* in different alkali metal hydroxide solutions showed that the solubility of K(2)FeO(4) was diminished in KOH and CsOH and similarly in LiOH and NaOH solutions. The solubility product constant (K(sp)*), calculated from the solubility measurements, was given as log K(sp)* = 18.10 - 6055/T - 17.535X(K+)(0.5) + 4.785X(K+) + 2171X(K+)(0.5)/T (sigma = 0.061). The heat of dissolution (Delta H(diss)) decreased linearly with an increase in the mole fraction of K(+) ions. The significance of the solubility results is briefly discussed.