The metastable phase equilibrium in the quaternary system NaCl + KCl + Na2CO3 + K2CO3 + H2O was Studied at 273 K using an isothermal evaporation method. The solubility and the density of the equilibrated Solution were determined. On the basis of the solubility data, the metastable phase diagram and the water diagram were plotted. The phase diagram Consists of three invariant points, seven univariant Curves, and five metastable crystallization fields. Among the three invariant points, points E-1 and E-3 are a Commensurate invariant point type, while point E, is a incommensurate invariant point type. The three cosaturated salts for points E-1, E-2, and E-3 are NaCl + KCl + Na2CO3 center dot 10H(2)O, KCl + Na2CO3 center dot 10H(2)O + KNaCO3 center dot 6H(2)O, and KCl + K2CO3 center dot 3/2H(2)O + KNaCO3 center dot 6H(2)O, respectively. The five metastable crystallization fields correspond to the single salts potassium chloride (KCl), sodium chloride (NaCl), sodium carbonate decahydrate (Na2CO3 center dot 10H(2)O), potassium carbonate sesquihydrate (K2CO3 center dot 3/2H(2)O), and a hexahydrate double salt of potassium and sodium carbonate (KNaCO3 center dot 6H(2)O). Sodium carbonate decahydrate (Na2CO3 center dot 10H(2)O) is the only crystallization form of sodium carbonate at 273 K in this system; the monohydrate (Na2CO3 center dot H2O) and heptahydrate (Na2CO3 center dot 7H(2)O) are not formed.