The solubility of the quaternary system sodium carbonate + ethanol + hydrogen peroxide + water was determined at 293.15 K using Schreinemaker's wet residue method. The phase diagram was constructed according to the Jneck method. There were three solid phases formed in the quaternary system, which corresponded to Na2CO3 center dot 10H(2)O, Na2CO3 center dot 1.5H(2)O(2)center dot H2O, and Na2CO3 center dot 2H(2)O(2)center dot H2O. The crystalline region of Na2CO3 center dot 1.5H(2)O(2)center dot H2O was larger than that of the other two solids, Na2CO3 center dot 10H(2)O and Na2CO3 center dot 2H(2)O(2)center dot H2O. Adding the ethanol into the Na2CO3 + H2O2 + H2O system could promote further crystallization and recover a greater fraction of Na2CO3 center dot 1.5H(2)O(2)center dot H2O. The concentrations of sodium carbonate and hydrogen peroxide in the mother liquor after separation from Na2CO3 center dot 1.5H(2)O(2)center dot H2O would be lowered as a result of the presence of ethanol. The solubility data and the phase diagram of the quaternary system could provide the fundamental basis and serve as a guide for the preparation of sodium percarbonate.