Activity of water in aqueous solutions containing sulfuric acid and uranyl sulfate has been determined at 298.15, 310.65, and 323.15 K by means of the apparatus AQUALAB 4TE. The data obtained, along with literature information about activity of water and solid–liquid equilibria in the H2O–UO2SO4 and H2O–H2SO4–UO2SO4 systems, were used to develop a thermodynamic model of the ternary system mentioned. The resulting model, employing Pitzer–Simonson–Clegg (PSC) equations to represent the properties of a liquid phase, is valid within a wide range of temperatures and concentrations. Apart from introducing novel experimental data and a thermodynamic model, the reported study discusses advantages and disadvantages of the PSC-based thermodynamic descriptions of the H2O–H2SO4 subsystem presented in the literature.