The values of enthalpy change ΔH1=-44.3 kJ/mol and entropy change ΔS1=-94.6 J/mol in the gas phase addition of a water molecule to a sulfuric acid molecule and entropy change ΔS2=-145 J/mol in hydration of a sulfuric acid monohydrate are obtained by a fitting procedure. The procedure uses experimental data from the literature on total pressure of sulfuric acid hydrates and free molecules. The enthalpy change in monohydrate hydration is taken, based on the results of ab initio calculations given in literature, to be ΔH2=ΔH1-10 kJ/mol. The estimate of covariance matrix of uncertainties of these values is obtained. The standard errors of estimates of ΔH1, ΔS1, and ΔS2 are 43%, 63%, and 45% of the above values, respectively. Uncertainties are strongly correlated. The sulfuric acid hydration model is proposed for geophysical applications. The model converts total acid to free acid in accordance with experimental measurements and increases the predicted values of sulfuric acid-water nucleation rate by 102--107 times compared with the classical liquid drop hydrate model. ¿ 2000 American Geophysical Union