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The solubility was studied for compounds in the system Zr(SO4)2–K2SO4–H2SO4–H2O in the section at 10 wt % H2SO4 under isothermal conditions at 25°C in the cuts at a constant solvent content of 60 wt % (H2O + H2SO4). Equilibrium in the system was reached for 16 h while continuously stirring. In the section at 10 wt % H2SO4, equilibrium solid phases are the compounds K2Zr(SO4)3 ⋅ 2H2O, K3Zr(OH)(SO4)3 ⋅ 2H2O, K6Zr(SO4)5 ⋅ 3H2O, and β-K2SO4. The recovered compounds were investigated by X-ray powder diffraction, crystal optical, thermal analyses and IR spectroscopy. It was found that the solubilities of the potassium zirconium sulfates are much lower than those of the sodium zirconium compounds. The results are of practical importance for sulfuric acid treatment of eudialyte, a zirconium-containing mineral, huge reserves of which occur in the Kola Peninsula. [ABSTRACT FROM AUTHOR]