Recent studies have suggested that the rates of reactions that occur on and in sulfuric acid solutions depend strongly on the activities of H2O and H+. To simplify the application of formulations for calculating rates of these reactions and thus facilitate their use in photochemical models of the atmosphere, this letter presents a parameterization for the activity of H+ based on the results of a comprehensive, but computationally intensive, thermodynamic model. The parameterization deviates from values given by the full thermodynamic model by <2% for temperatures between 186 and 326 K and sulfuric acid concentrations between 10 and 80 weight percent. ¿ 1998 American Geophysical Union