Isobaric vapor–liquid equilibrium (VLE) data have been measured for the ternary system (ethylbenzene + styrene + sulfolane) and the three constituent binary systems under vacuum [(5, 10, and 20) kPa]. The VLE data of the binary system (ethylbenzene + styrene) measured in this work are thermodynamically consistent according to the Herington area test and the point test method contrary to the low pressure VLE data about this system available in the literature. The binary VLE data were described well by the nonrandom two-liquid (NRTL) model. The relative volatility of the system (ethylbenzene + styrene) increases in the presence of sulfolane from 1.4 up to values of 2.2. The ternary system could only be well-correlated when using the ternary VLE data in combination with the binary VLE data as input for the regression of the NRTL binary interaction parameters.