Isobaric vapor-liquid equilibria for the ethylbenzene + p-xylene system at 40.00, 66.66, and 93.32 kPa have been measured using a modified Rogalski-Malanowski equilibrium still. Parameters for the NRTL equation are determined from the experimental vapor-liquid equilibrium data. In addition, the number of columns and the reflux ratio needed for separation by distillation are estimated. The separation performance can be partially evaluated, even if the mixture is assumed to be an ideal solution. However, we find that the temperature distribution in the distillation column differs, and that consideration of the activity coefficients affects the selection of the feed location.