The effect of amino acids such as asparagine, valine, glutamic acid and glycine on silver ion activity as a olefin carrier in membrane consisting of silver ions dissolved in poly(2-ethyl-2-oxazoline) (POZ) has been investigated: The presence of amino acids leads to the weakened interaction of the silver ion with its counteranion resulting from the electrostatic interaction between counteranion and NH3+ of amino acids, as confirmed by FT IR spectra: As a result, the mixed gas permeances and selectivities of propylene/propane through POZ/AgBF4 or POZ/AgNO3 membranes intriguingly increased upon addition of the amino acids. However, the improvement of separation performance is strongly dependent on the type and the concentration of amino acids. Among the amino acids tested, the presence of asparagine containing amide group was found to improve the separation performance of the membranes most significantly. On the other hand, the performance of membrane with lysine containing amine group was the lowest, indicating the amine is not the effective ligand for silver ion in facilitated olefin transport membranes.. (C) 2004 Elsevier B.V. All rights reserved.