Ultrasonic velocities in ternary systems of triethylamine in aqueous sodium chloride solutions have been measured at 25 °C, for sodium chloride molalities ranging from 0 to 0.3 mol kg^–1. The concentration of triethylamine was varied from 0 to ca. 0.35 mol kg^–1. The concentration of triethylamine was varied from 0 to ca. 0.35 mol kg^–1 in pure water and in each of the four concentration-fixed NaCl solutions investigated. Isentropic compressibilities, κ_S, were calculated from ultrasonic velocity and density data. Excess molar isentropic compressions, K^E_S,m were estimated and their variations with the amine mole fraction were fitted to the Redlich–Kister equation. Negative K^E_S,m values were obtained for all the systems in the concentration range studied. Evaluation of the triethylamine limiting partial molar isentropic compression, K^∞_S,Et3N was made for each system, using dK^E_S,m/dX_Et3N derivatives. Negative K^∞_S,Et3N values were obtained, increasing in absolute value up to 0.09 mol kg^–1 NaCl concentration and then steeply decreasing. Apparent molar isentropic compressions, K_{S, ϕ}, of Et₃N were also calculated and plotted for all the solvents studied.

The interpretation of the results, derived from experimental ultrasonic velocities, was made in terms of the important roles of the ions and the Et₃N molecule in determining the water structure. The results support and complement the conclusions made in Part 1 of this work, which were based on volumetric data.