Separation of linear and branched alkane isomers via selective permeation through a composite membrane is disclosed. The separation layer in the composite membrane is fabricated from a blend of at least two different fluoropolymer compositions, A and B, in which composition A has a normal-alkane isomer permeability that is greater than composition B. Composition B has a normal alkane to branched-alkane isomer selectivity that is equal or greater than composition A. The separation layer in the composite membrane has a normal-alkane permeability that is greater than composition B and a normal-alkane to branched alkane isomer selectivity that is greater than composition A.