Equilibrium data for the NaOH-NaNO3-Na2CrO4-H2O quaternary system from (313 to 393) K were measured, and the phase diagram at 393 K was constructed, with the crystallization areas in the phase diagram discussed in detail. In addition, the solubility isotherms of Na2CrO4 and NaNO3 in NaOH solutions were plotted. The solubility of Na2CrO4 in NaOH solutions was compared with that in NaOH solutions saturated with NaNO3. On the basis of the phase diagram, a strategy for effective separation of Na2CrO4 and NaNO3 from the NaOH-NaNO3-Na2CrO4-H2O quaternary system has been proposed.