Liquid-liquid equilibria (LLE) were determined experimentally for the system water + 2-heptanone + caprolactam + ammonium sulfate at 20, 40 and 60degreesC. The equilibrium data show the salt and temperature effect on the distribution of caprolactam: at higher salt concentrations and at higher temperatures more caprolactam is transferred to the organic phase. A three-liquid phase region was found in the system and measurements were carried out to determine the three-phase region. The location of the three-phase triangles is largely dependent on the salt concentration and follows the pattern found in literature for surfactant systems. The three-liquid phase equilibrium data presented here, in combination with equilibrium data from literature, were modeled with the modified extended electrolyte non-random two-liquid (NRTL) model. The model was found able to give a satisfactory description of three-liquid phase equilibria. For the two-liquid phase equilibria, a good representation of the experimental data could be obtained as well, using the same binary parameters as for the liquid-liquid-liquid equilibria (LLLE) data.