Polymer/silver-ion pi-complex membranes consisting of poly(hexamethylenevinylene) (PHMV) and silver tetrafluoroborate exhibit unusually high separation performance for olefin/olefin and olefin/paraffin mixtures. The formation of pi complexes between silver ions and unsaturated C=C bonds of PHMV has been confirmed with wide-angle X-ray scattering, differential scanning calorimetry, and X-ray photoelectron spectroscopy. Fourier transform infrared and ultraviolet spectroscopy studies have revealed that silver ions make pi complexes with olefins such as 1,3-butadiene, propylene, and ethylene. Of these three olefins, 1,3-butadiene has the highest binding affinity with silver ions in dissolved in PHMV, and this results in its higher solubility and permeance. Therefore, the pi-complex membranes exhibit unusually high separation performance for olefin/olefin and olefin/paraffin mixtures. (c) 2006 Wiley Periodicals, Inc.