In this investigation, the mutual solubilities for the ternary Na2C2O4 + H2O2 + H2O system were determined at (283.15 and 293.15) K. The phase diagrams of the system were constructed based on the measured solubility. The compound Na2C2O4.H2O2 was confirmed by Schreinemaker's wet residue method. In addition, the density of the system was obtained. At (283.15 and 293.15) K, two solid phases were formed in the ternary Na2C2O4 + H2O2 + H2O system, which corresponded to Na2C2O4 and Na2C2O4.H2O2. The phase diagrams of the ternary system were similar at different temperatures; the solubilities of Na2C2O4 and Na2C2O4.H2O2 in water increased slightly with increasing temperature. The crystalline region of the compound Na2C2O4.H2O2 decreased as the temperature increased, the temperature having little effect on the yield of Na2C2O4.H2O2.