In an isomerization process where the isomerization effluent (108) is fractionated in a deisohexanizer (116) to provide an overhead (118) containing dimethylbutanes and a higher boiling fraction (122) containing normal hexane, the higher boiling is contacted with a selectively permeable membrane (124) to provide a retentate containing methylcyclopentane (128). If desired, the normal hexane-containing permeate can be recycled for isomerization. The preferred membranes are sieving membranes having a C.sub.6 Permeate Flow Index of at least 0.01 and a C.sub.6 Permeate Flow Ratio of at least 1.25:1.