Equilibria data for the NaOH-Na2MoO4-Na2SO4-H2O quaternary system at (313.15 and 403.15) K were measured with an isothermal method. According to the experimental results, dry-salt and water phase diagrams were constructed. It was found that there are two invariant points, five univariant curves, and four crystallization zones corresponding to Na2MoO4, Na2SO4, Na2MoO4 center dot 2H(2)O, and NaOH center dot H2O (NaOH at 403.15 K). In addition, the solubility of Na2MoO4 in NaOH solution was compared with that in NaOH solution saturated with Na2SO4 at both temperatures, while the solubility of Na2SO4 in NaOH solution saturated with Na2MoO4 at 313.15 K was compared with that at 403.15 K.