The isothermal evaporation method was employed to investigate the metastable phase equilibria in the quaternary system Na+ + K+ + CO32- + B4O72- + H2O at 273.15 K The solubility and density of the metastable equilibrated solution were determined 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 metastable equilibrium phase diagram, water-content diagram, and density-composition diagram of the quaternary system were plotted The phase diagram consists of three invariant points, seven univariant curves, and five crystallization fields corresponding to sodium borate decahydrate (Na2B4O7 center dot 10H(2)O), potassium borate tetrahydrate (K2B4O7 center dot 4H(2)O), sodium carbonate decahydrate (Na2CO3 center dot 10H(2)O), potassium carbonate sesquihydrate (K2CO3 center dot 3/2H(2)O), and a potassium and sodium carbonate double salt (K2CO3 center dot Na(2)CO(3 center dot)12H(2)O). This quaternary system is of a complex type Na2CO3 center dot 10H(2)O is the only crystalloid form of sodium carbonate in the given system at 273.15 K Sodium borate has the largest crystallization field and can be easily separated from the solution in this system at 273 15 K using an evaporation method If borate needs to be separated as K2B4O7 center dot 4H(2)O as much as possible, a rise in temperature is necessary.