The solubility of potassium ferrate (K2FeO4) was measured in aqueous solutions of NaOH and KOH of total concentration 12 M containing various molar ratios of KOH:NaOH in the range 12:0 to 3:9. Several analytical methods were tested for the determination of ferrate concentration. The final method chosen consisted of potentiometric titration of the ferrate sample with an alkaline solution of As2O3. The assumption was made that ferrate dissociates in concentrated KOH solutions predominantly to KFeO4-. The solubility constant, S, defined as the product of the molar concentration of the potassium ion, K+, and the ferrate anion, KFeO4- was found to be 0.044 +/- 0.006 mol(2) dm(-6) for 20 degrees C, 0.093 +/- 0.004 mol(2) dm(-6) for 40 degrees C and 0.15 +/- 0.09 mol(2) dm(-6) for 60 degrees C. From these results the heat of dissolution of K2FeO4 was calculated as -14.3 kJ mol(-1). At 60 degrees C the enhanced decomposition of the ferrate at the higher temperature led to a greater deviation in solubility values compared with data for either 20 degrees C or 40 degrees C.