The phase equilibria of the ternary system (Li2CO3-Na2CO3-H2O) at 278.15 <= T/K <= 308.15 K were investigated by isothermal dissolution equilibrium. On the basis of the experimental data, the phase diagrams and densities versus mass fraction (100w) of sodium carbonate were plotted. For the ternary system, there is one invariant point, two univariant curves, and two crystallization regions corresponding to lithium carbonate (Li2CO3, LC) and sodium carbonate decahydrate (Na2CO3 center dot 10H(2)O, NC10) or thermonatrite (Na2CO3 center dot H2O, NC). No solid solution or double salt was found in the ternary system at the experimental temperature; the ternary system was a simple eutectic type. In a comparison of the phase diagrams from 278.15 to 308.15 K in the system, the crystallization region of NC10 existed at 278.15, 288.15, and 298.15 K, and the equilibrium solid phase corresponding to sodium carbonate was dehydrated and transformed to thermonatrite at 308.15 K. The crystallized area of lithium carbonate expands with increasing temperature, while the crystallized areas of sodium carbonate demonstrate an opposite trend. The components of Na2CO3 in solution have a common ion and salting-in effect to Li2CO3, and the salting-in effect increases with increasing concentration. This indicates that a large amount of excess sodium carbonate could not improve the yield of lithium in the carbonated precipitated lithium process. The density of this ternary system changes regularly with the changing concentration of sodium carbonate, and it was represented by empirical equations as functions of concentration.