The vapor pressures of the 1,4-dioxane + tetrabutylammonium nitrate and water + tetrabutylammonium nitrate systems have been measured at different salt molalities. The vapor pressures of the 1,4-dioxane + water + tetrabutylammonium nitrate system have also been measured at I I constant salt molalities (0.100, 0.250, 0.500, 0.750, 1.000, 1.500, 2.000, 2.500, 3.000, 3.500, and 4.000) mol.kg(-1). These systems have been studied at (288.15, 298.15, and 308.15) K with a modified Othmer still. The electrolyte nonrandom two-liquid model was used for the calculation of the activity coefficients of solvents in the binary systems.