The metastable phase equilibria of the quinary system NaCl + Na2CO3 + Na2B4O7 + KCl + K2CO3 + K2B4O7 + H2O were studied at 273.15 K using an isothermal evaporation method. The solubilities and densities of the equilibrated solution in this system were determined. On the basis of the experimental data, the stereo metastable phase diagram and the projected phase diagram saturated with the salt Na2B4O7 of the quinary system at 273.15 K were constructed. In the space diagram there are seven crystallization fields corresponding to the crystalline phase regions of KCl, NaCl, K2B4O7 center dot 4H(2)O, Na2B4O7 center dot 10H(2)O, Na2CO3 center dot 10H(2)O, K2CO3 center dot 3/2H(2)O, and K2CO3 center dot Na2CO3 center dot 12H(2)O. The projected phase diagram saturated with sodium borate consists of five crystallization fields, seven univariant curves, and three invariant points. The five crystallization fields correspond to NaCl, KCl, K2B4O7 center dot 4H(2)O, Na2CO3 center dot 10H(2)O, and K2CO3 center dot Na2CO3 center dot 12H(2)O. The forms of sodium and potassium borate are Na2B4O5(OH)(4)center dot 8H(2)O and K2B4O3(OH)(4)center dot 2H(2)O, respectively. The forms of potassium carbonate, sodium carbonate, and double salt are K2CO3 center dot 3/2H(2)O, Na2CO3 center dot 10H(2)O, and K2CO3 center dot Na2CO3 center dot 12H(2)O; comparing with the phase diagram of the subsystems containing sodium carbonate and potassium carbonate, they have the same crystallization forms.