In this investigation, the mutual solubility for the ternary sodium 3-nitrobenzenesulfonate + sodium 4-nitrobenzenesulfonate + water system was determined at (283.15, 303.15, and 323.15) K. The phase diagrams of the system were constructed based on the measured solubility. The solid phases formed in the studied system were confirmed by Schreinemaker's wet residue method. In addition, the density of the equilibrium liquid phase was obtained. At (283.15, 303.15, and 323.15) K, there are two pure solids formed which correspond to sodium 3-nitrobenzenesulfonate and sodium 4-nitrobenzenesulfonate in the ternary sodium 3-nitrobenzenesulfonate + sodium 4-nitrobenzenesulfonate + water system. Besides, the adduct of sodium 3-nitrobenzenesulfonate with sodium 4-nitrobenzenesulfonate, in which the mole ratio of the two compositions is 1:1, was formed at 323.15 K.