The isopiestic molalities for the systems LiBr-H2O, CaBr2-H2O and LiBr-CaBr2-H2O were measured at 373.15 K with the isopiestic method using the Oak Ridge National Laboratory (ORNL) high-temperature gravimetric isopiestic apparatus. The osmotic coefficients were determined based on the lithium chloride and sodium chloride reference standards. The vapor pressure diagrams and osmotic coefficient diagrams at different mole fraction for LiBr were plotted. The calculated vapor pressure with water activities agree well with the experimental data. On the basis of Pitzer original and its extended model, the Pitzer binary parameters beta((0)), beta C-(1)((0)) and C-(1) for the systems LiBr-H2O and CaBr2-H2O and mixing parameters of theta(Li,ca) and psi(Li,Ca,Br) were fitted with the measured osmotic coefficients. The calculated osmotic coefficients for systems LiBr-H2O and CaBr2-H2O agree well with the experimental data. The agreement between the calculated water activities and experimental results for the ternary system indicates that the parameters obtained in this work are reliable for water activity calculation. The thermodynamic data obtained in this study are essential for development of universal thermodynamic models for brine systems containing lithium bromide and calcium bromide. (C) 2018 Elsevier Ltd.