The objective for this research was to employ the extended adsorption isotherm (EAI) to develop a predictive relationship between the activity of H2O (aw) and the mean ionic activity (a +/-) of aqueous HNO3. The EAI model is a calculative approach to the estimation of solutions for activities of highly non-ideal chemical systems and is the collective efforts of Stokes and Robinson, Abraham, and Ally and Braunstein. From previous works of the HNO3-H2O system by Rains et al., a predictive model of relevant aqueous phase activities was developed that only needs knowledge of composition and temperature of the HNO3 solution.