Equilibrium data of the NaOH-NaBO2-Na2CO3-H2O quaternary system at 30 degrees C, 60 degrees C, and 100 degrees C were measured by an isothermal method. The phase diagrams at 30 degrees C and 100 degrees C were constructed, and the crystallization zones were discussed. In the phase diagram, the quaternary system has two invariant points, five univariant curves, and four crystallization zones corresponding to NaBO2 center dot 4H(2)O (NaBO2 center dot 2H(2)O at 100 degrees C), NaBO2 center dot 2H(2)O (NaBO2 center dot 1/3H(2)O at 100 degrees C), Na2CO3 center dot H2O, and NaOH center dot H2KO (NaOH at 100 degrees C). In addition, the solubility isotherms of NaBO2 in Na2CO3-saturated NaOH solution and the solubility isotherms of Na2CO3 in NaBO2-saturated NaOH solution were plotted. The solubility of Na2CO3 in NaOH solution was compared with that in NaBO2-saturated NaOH solution. On the basis of the solubility data, a strategy for the effective separation of NaBO2 and Na2CO3 from the NaOH-NaBO2-Na2CO3-H2O quaternary system was proposed.