Thermodynamic modeling of the sulfuric acid-water-sulfur trioxide system is of great interest to the industry. The recently developed symmetric electrolyte NRTL activity coefficient model is applied to develop a comprehensive thermodynamic model for the sulfuric acid system over the whole concentration range from pure water to pure sulfuric acid to pure sulfur trioxide with temperature up to 773 K. The model takes into account partial dissociations of sulfuric acid and bisulfate ion, hydration of hydronium ion, decomposition of sulfuric acid to water and sulfur trioxide, and formation of disulfuric acid from sulfuric acid and sulfur trioxide in the concentrated sulfur trioxide region. Excellent matches between model correlations and available literature data are achieved for vapor-liquid equilibrium, osmotic coefficient, liquid phase speciation, heat of dilution, heat of mixing, heat of solution, and liquid heat capacity of the sulfuric acid-water-sulfur trioxide system.