A thermodynamic model is presented for nitric acid extraction by tri-n-butyl phosphate (TBP). This model is based on the formation of the organic phase species: TBP·HNO3 and (TBP)2.·HNO3. The model works successfully at TBP concentrations of 5 to 100 vol% and was found to be effective at predicting the extraction of HNO3 from HNO3/NaNO3 and HNO3/ LiNO3 solutions. Within the TBP concentration range of 5 to 30%, a single set of extraction constants was sufficient to fit extraction data. Stoichiometric activity coefficients of nitric acid in HNO3/NaNO3 and HNO3/LiNO3 mixtures were calculated using a model developed by Bromley.