The densities and excess molar volumes (V-E) for tri-n-octylamine (TOA) + propionic acid, TOA + diluent, and propionic acid + diluent binary systems were measured under the full range of composition at 298.15 K, respectively. The excess molar volumes of TOA + propionic acid + diluent ternary systems were also measured at 298.15 K. Benzene, toluene, p-xylene, ethanol, hexane, and cyclohexane were used as the diluent. The V-MAX(E) of TOA + propionic acid is found to be about 6 cm(3) mol(-1) at x(A) = 0.75. The maximum values of excess molar volume (V-MAX(E)) for the ternary systems showed smaller values than that of TOA + propionic acid.