The salt effect on the partially miscible system 1-butanol and water containing penicillin G potassium salt was investigated at low pressures using the ebulliometric method. The mean evaporation coefficient, f of the small ebulliometer used was determined, and the reliability of the ebulliometric method was examined. Phase equilibrium data for the partially miscible system 1-butanol + water without salt and with different salt concentrations at 8.00, 5.33, and 2.67 kPa were measured with the ebulliometer. These data were used to calculate vapor-liquid equilibrium (VLE) compositions using the NRTL model and the modified NRTL model, respectively. vapor-liquid-liquid equilibrium (VLLE) compositions at certain pressure for the binary system without salt and with the salt were also calculated using the obtained NRTL parameters. Some of the results of the calculation were verified by experiments with a glass equilibrium still and gas chromatography analysis and showed good agreement.It was found that the salt effect of the penicillin G potassium salt on the 1-butanol + water binary system at low pressures causes the azeotropic temperature to rise slightly and increases the mutual solubility of the two components. When the penicillin G potassium salt concentration reached 10 wt.%, the 1-butanol + water system becomes completely miscible. (C) 2003 Elsevier B.V. All rights reserved.