Standard state thermodynamic properties for completely dissociated aqueous barium sulfate, BaSO4(aq), were fixed by an ionic additivity relationship using the data from other completely dissociated electrolytes, barium chloride, sodium sulfate, and sodium chloride, from 298.15 to 598.15 K and at steam saturated pressure, p(sat). The solubility of barium sulfate in pure water was then predicted a priori from 273.15 to 598.15 K and pressures up to 140 MPa, and the results were compared with the literature data. These predicted solubility values for barium sulfate in pure water, when corrected for ion pair association of Ba2+(aq) ion with SO42-(aq) ion and hydrolysis of SO42-(aq) ion, are in very good agreement with the literature data to well within the uncertainties of the experimental data at all temperatures and pressures considered. Using the experimental data from this study and auxiliary data from the literature, the logarithm: of the molar dissociation constants of BaSO40 associated ion pair were also calculated up to 598.15 K and lip to 150 MPa.