Liquid-liquid equilibrium and tie line data for the nitric acid/phosphoric acid/water/tri-n-octylamine (TOA) system have been determined at 298.15 K for HNO3 mass fractions of 0.028 to 0.26 and H3PO4 mass fractions of 0.00 to 0.19. The distribution coefficients of nitric acid (D-1) and water (D-3) were studied, and the separation factors beta(HNO3/H3PO4) was also investigated. The results show that TOA is very selective, since only small quantities of water and phosphoric acid are present in the organic phase, which indicates the good ability of TOA to extract nitric acid from nitric acid/phosphoric acid mixtures. The experimental tie line data were correlated using the nonrandom two-liquid (NRTL) model. TOA can be used as an effective extractant to separate nitric acid from the HNO3/H3PO4/H2O system.