An internally staged hollow fiber polysulfone membrane is used to generate a permeate stream enriched in C4H10 from a feed mixture of CH4, C2H6, C4H10, CO2 and Nz. Experimental measurements are made as a function of the feed pressure and the reject how rate. The area ratio of the first and second membrane stages are kept constant as well as the feed composition and the pressure ratio of the first permeate and the feed streams. The plug flow model is used to simulate the separation performance of the multicomponent mixture in the internally staged membrane system. The analysis is focused on determining the local optimum mole fraction for the C4H10 in the second permeate stream over a wide range of operating conditions. The local optimum is determined by varying the feed pressure, first membrane area, pressure and area ratios, and the stage cut. Results show that the local optimum occurs at small membrane area and area ratio, low stage cut and pressure ratio, and high feed pressures. Good agreement is found between measurements and model predictions for the C4H10 mole fraction in the second permeate stream and the stage cut.