Isopiestic molalities, vapor pressures and osmotic coefficients of aqueous Li 2B 4O 7 solutions from 0.027 5 to 2.939 7 mol/kg were determined at 298.15 K by using an improved apparatus. The rule of the changes in vapor pressure over aqueous Li 2B 4O 7 solutions was revealed, which indicated that the bond of borate anions with water are better than that of NaCl. The different formation reaction equilibria for the species containing boron of boric acid and borate anions B(OH) - 4, B 3O 3(OH) - 4 and B 4O 5(OH) 2- 4 in Li 2B 4O 7(aq) solutions were taken into account. The stoichiometric equilibrium constants for the formation reactions of the boron species and the ion-interaction parameters were estimated simultaneously by using Pitzer equations of osmotic coefficient with minor modifications. Sixteen parameters were obtained with a standard deviation 0.005 5 for the ion-interaction model.