Condensation of VOCs in VOC-enriched permeate streams at room temperature in membrane-based VOC/N-2 separation make accurate measurement of membrane separation performance difficult. Membrane performance is important for obtaining fundamental data for industrial applications. Therefore, in this work, a modified constant volume/variable pressure method was developed to avoid the effects of VOC condensation in the permeate and to accurately determine membrane performance for the separation of VOC/N-2 mixtures. Development of the method is discussed and validated by measuring the separation performances of p-xylene/N-2 mixtures using a PDMS/ceramic composite membrane, which showed good reliability. Effects of feed concentration, feed flow rate, and temperature on separation performance were investigated by this method for the separation of representative VOC pollutants such as acetone, p-xylene, ethanol, and ethyl acetate. The perm abilities of VOC and N-2 and their interaction was easily obtained, which open the possibility of the investigation of the interactions between VOC and N-2 during vapor permeation.