In a normal butane isomerization process where the isomerization effluent (108, 114) is fractionated in a deisobutanizer (116) operated such that a lower boiling fraction (118) is provided containing at least 80 mass-% isobutane and a higher boiling fraction (122) containing normal butane and at least 10 mass-% isobutane. The higher boiling fraction (122) is contacted with a selectively permeable membrane (124) to provide a permeate (126) containing normal butane-containing and a retentate (128) containing at least 80 mass-% isobutane. The preferred membranes are sieving membranes having a C.sub.4 Permeate Flow Index of at least 0.01 and a C.sub.4 Permeate Flow Ratio of at least 1.25:1.