The properties of fluoride in wet phosphoric acid processes were of interest us to understand the processes and roles that fluorosilicic acid plays in phosphoric acid solutions. The literature data indicate that some deficiencies exist in the thermodynamic properties of hexafluorosilicic acid aqueous solutions. In this investigation, the water activity and osmotic coefficients of H2SiF6(aq) were determined in the temperature range from T = 298.15 to 35315 K. The measurements of the water activity were performed at molalities from 0.10 to 3.00 mol kg(-1) of H2SiF6(aq) using the hygrometric method. The modelling approach based on the Pitzer model was developed to determine the thermodynamic properties. From these measurements, the ion interaction parameters in binary solution were evaluated at different temperatures ranging from 298.15 K to 353.15 K and were used to predict the solute activity coefficients. The development of a model for solution behaviour was also employed to determine the excess molal Gibbs energy, enthalpy, entropy and heat capacity of the H2SiF6 aqueous solution within the temperature range from 298.15 K to 353.15 K. (C) 2015 Elsevier B.V. All rights reserved.