The phase equilibria in the quaternary system Na(2)SO(4) + K(2)SO(4) + Na(2)CO(3) + K(2)CO(3) + H(2)O were studied at 273.15 K using an isothermal evaporation method. The solubilities and densities of the equilibrated solution were determined experimentally. The crystalloid forms of the solid phase were determined using chemical analysis and an X-ray diffraction method. On the basis of the experimental data, the isothermal equilibrium phase diagram, water content diagram, and density composition diagram of the quaternary system were plotted. The phase diagram consists of four invariant points, nine univariant curves, and six crystallization fields corresponding to sodium sulfate decahydrate (Na(2)SO(4)center dot 10H(2)O), potassium sulfate, sodium carbonate decahydrate (Na(2)CO(3)center dot 10H(2)O), potassium carbonate hydrate (K(2)CO(3)center dot 3/2H(2)O), potassium and sodium carbonate double salt (K(2)CO(3)center dot Na(2)CO(3)center dot 12H(2)O), and sulfate and carbonate double salt of sodium (2Na(2)SO(4)center dot Na(2)CO(3)). This quaternary system is of a complex type. Two kinds of double salts are formed. Na(2)CO(3)center dot 10H(2)O is the only crystalloid form of sodium carbonate in the given system at 273.15 K. Na(2)CO(3)center dot 7H(2)O and Na(2)CO(3)center dot H(2)O are not formed.