In this paper, a new solid polymer electrolyte PEI/Pebax2533/AgBF4 composite membrane (PEI: polyetherimide; Pebax2533: Polynylon12/tetramethylene oxide block copolymer. PA12-PTMO) is prepared with complexing method. When the external surface of Pebax2533 layer is immersed into AgBF4 solution, silver salt transfers into Pebax2533 layer according to the complexation with ether oxygens in Pebax2533 backbone, and an ultrathin solid polymer electrolyte layer is formed. When AgBF4 concentration is less than the threshold silver concentration C*(4.5 mol/L), because of low silver salt content in the solid polymer electrolyte layer, no carrier-facilitated transport phenomenon occurs. But for PEI/Pebax2533/AgBF4 composite membrane prepared with AgBF4 concentration above C*(4.5 mol/L), the percolation channels for carrier-facilitated transport are formed and the preferential adsorbed propylene molecules can transfer across the membrane easily, which result in an increase of propylene permeance and a sharp decrease of propane permeance. According to TEM observation, an ultrathin solid polymer electrolyte layer of 100-200 nm thickness exists in the external surface of the Pebax2533 layer of PEI/Pebax2533/AgBF4 composite membrane. (C) 2010 Elsevier B.V. All rights reserved.