Phase equilibrium of ternary system K2O-Al2O3-H2O is the key in realizing seeded precipitation process of aluminum hydroxide product from alkaline leaching solution in the alunite tailings metallurgical industry. However, little work has been carried out about the solubility of the K2O-Al2O3-H2O system, especially for the concentrated alkali solution region. The corresponding phase equilibria at 323.15, 333.15, 343.15, and 353.15 K were researched by using the isothermal dissolution method and the Pitzer ion-interaction model. The calculated Al2O3 center dot 3H(2)O solubilities corresponding to the crystallization region of Al2O3 center dot 3H(2)O in the ternary system at each temperature basically agree with experimental values when K2O concentration is below 30 wt %. Furthermore, the ternary system at each temperature contains two invariant points, three univariant isothermal dissolution curves, and five crystallization fields corresponding to Al2O3 center dot 3H(2)O (gibbsite), K2O center dot Al2O3 center dot 3H(2)O, KOH center dot H2O, Al2O3 center dot 3H(2)O + K2O center dot Al2O3 center dot 3H(2)O, and K2O center dot Al2O3 center dot 3H(2)O + KOH center dot H2O. According to the phase diagrams of the ternary system, it can be indicated that the low alkali concentration and temperature are in favor of the crystallization of Al2O3 center dot 3H(2)O, while the high alkali concentration and low temperature are beneficial to the crystallization of K2O center dot Al2O3 center dot 3H(2)O.