In an isomerization process where the isomerization effluent (114) is fractionated in a deisohexanizer (116) to provide a lower boiling fraction (122) containing dimethylbutanes, a higher boiling fraction containing methylpentanes and a bottoms stream (120) containing normal hexane and heavies, the bottoms stream is contacted with a selective permeation membrane (124) to provide a normal hexane-containing permeate (128) suitable for recycle to the isomerization. The energy consumption of the deisohexanizer (116) can be reduced without adversely affecting the octane rating of the overhead (122).