Solubility isotherms are reported for the binary systems, NaNO3 + Na2SO4, and NaNO2 + Na2SO4, in aqueous solution and in I m NaOH and 3 m NaOH solutions. Measurements were conducted at both (25 and 50) degrees C for the NaNO2 + Na2SO4 system and at (25, 35, and 50) degrees C for the NaNO3 + Na2SO4 system. The presence of either sodium nitrate or sodium nitrite in solution results in a reduction of the solubility of sodium sulfate. At 25 degrees C, a transition of the stable crystalline phase from sodium sulfate decahydrate to anhydrous sodium sulfate occurs as the amount of either sodium nitrate or sodium nitrite in aqueous solution increases. The presence of sodium hydroxide in solution also results in decreased solubility over the entire isotherm, with an increase in sodium hydroxide concentration giving rise to lower solubilities. In the NaNO3 + Na2SO4 system, the incongruent double salt, darapskite, NaNO3 center dot Na2SO4 center dot H2O, was identified as the stable crystalline phase over a portion of the solubility isotherm at all temperatures examined. TGA experiments were performed to determine the correct number of waters of hydration for darapskite.