A phase diagram for the NaOH-Al(OH)(3)-Na2CO3-H2O system at 393 K is constructed, and five crystallization zones are discussed in detail. With the increase of the concentration of sodium hydroxide, the Al2O3 solubility shows an initial rise followed by a downward trend, but the solubility of Na2CO3 decreases. The solubility of Na2CO3 is found to be not sensitive to the temperature and the Al2O3 concentration in the sodium aluminate solution. The study indicates a feasible evaporation process when the high alkali aluminate solution with a high molar ratio of sodium oxide to alumina (MR) was concentrated from 380 g.L-1 to 550 g.L-1 or even to higher concentrations.