Liquid-liquid equilibria of the quaternary system water + caprolactam + 1-octanol + ammonium sulfate at (25, 45, and 65) degrees C were determined. In our experiments, the distribution coefficient of caprolactam was usually higher than 1. With increasing temperature and salt content, the distribution coefficient of caprolactam increased. With increasing caprolactam content, the distribution coefficient became smaller. Compared to benzene, 1-octanol is nontoxic and has a much larger extraction capacity. Hence 1-octanol is a prospective substitute of benzene in caprolactam extraction. The extension of the Lyngby modified UNIFAC model was applied to represent the equilibrium data of systems without ammonium sulfate. CH2NHCOCH2 was defined as a new group, and new interaction parameters were determined from the equilibrium data of the ternary systems water + caprolactam + 1-octanol, water + caprolactam + 1-heptanol, water + caprolactam + benzene, and water + caprolactam + toluene. With the new interaction parameters, the equilibrium of the systems water + caprolactam + 1-heptanol + heptane and water + caprolactam + 1-heptanol + methyl cyclohexane were predicted. The prediction results agreed well with experimental value when the mass fraction of caprolactam is small.