This study deals with poly(ether block amide) (PEBA) composite membranes for propylene separation from nitrogen, which is relevant to the recovery of propylene from the degassing off gas during polypropylene manufacturing. The composite membranes, comprising of a thin layer of PEBA 2533 supported on microporous polysulfone substrate, were initially studied for pure gas permeation to evaluate the effects of pressure and temperature on the membrane permeance, and then they were investigated for the permeation of propylene/nitrogen mixture. It was found that the propylene permeance was affected by the presence of nitrogen, and vice versa, due to interactions between the permeating components. For the separation of propylene/nitrogen gas mixtures, both the permeance of propylene and the propylene/nitrogen selectivity were lower than those based on pure gas permeation. Semi-empirical correlations were developed to relate the permeance of a component in the mixture to the pressures and compositions of the gas on both sides of the membrane, and the performance of the membrane for propylene/nitrogen mixture separation at different operating conditions (e.g., temperature, pressure and feed composition) was analyzed in terms of product purity, recovery and productivity on the basis of cross flow model. Although the model calculations represented a conservative estimate on the separation performance, the results showed that the poly(ether block amide)/polysulfone composite membranes were promising for the separation of propylene from nitrogen.