The dissociation reaction of the bisulfate ion, HSO4-reversible arrow SO(4)2(-) + H+, is investigated in aqueous H2SO4 solutions with concentrations of 0.54-15.23 mol kg(-1) in the temperature range of 180-326 K using Raman spectroscopy. All investigated H2SO4 Solutions show a continuous increase in the degree of dissociation of HSO4- with decreasing temperature, in contrast to predictions from thermodynamic models of aqueous H2SO4 solutions. A Pitzer ion interaction model is used to derive a thermodynamically consistent formulation of the thermodynamic dissociation constant of the bisulfate ion, K-II(T), that is in agreement with the experimental data. The new formulation of K-II(7) is valid from 180 to 473 K. All ion interaction parameters and the corresponding parametrizations of the Pitzer ion interaction model are presented. Calculations with this model reveal significant differences in ion activity coefficients, water activities, water vapor pressure, and HCl solubilities, when compared to existing thermodynamic models of H2SO4/H2O solutions, in particular at lower temperatures.