Isopiestic molalities and water activities have been measured for aqueous Li2B4O7 solutions from (0.0275 to 2.9397) mol.kg(-1) at T = 298.15 K by isopiestic method, using an improved apparatus. These measurements have extended into the very dilute and the supersaturated molality region. Two types of osmotic coefficients of phi(S) and phi(E) were determined. phi(S) is based on the stoichiometric molalities of the solute Li2B4O7(aq), and phi(E) is based on equilibrium molalities based on consideration of the equilibrium speciation of H3BO3, B(OH)(4)(-), B3O3(OH)(4)(-), and B4O5(OH)(2)(-) in the Li2B4O7(aq) solutions. The stoichiometric equilibrium constants K-mi for the aqueous speciation reactions were estimated. Three types of representations of the osmotic coefficients for the (Li2B4O7 + H2O) system with the ion-interaction models based on Pitzer's equations of osmotic coefficients with minor modifications were presented: the model (I) for representation of the data of Os with four parameters based on considering the ion-interactions between two ionic species of Li+ and B4O72-, the model (II) for representation of the data of phi(E) based on considering the equilibrium speciation with 18 parameters, and the model (III) simplified from model (II) with six ion-interaction parameters estimated. The reasonable agreements between the experimental osmotic coefficient data and those calculated using the models above were obtained with the standard deviations of 0.0215, 0.0055, and 0.0150 for the three models, respectively. The thermodynamic osmotic coefficient properties for the complex system containing polymetric boron anions and lithium may be explained by use of the Pitzer ion-interaction model with minor modifications in combination with speciation reaction equilibria. (C) 2004 Elsevier Ltd. All fights reserved.