The facilitated transport membranes, AgClO4-CA (cellulose acetate) and AgBF4-CA were prepared and tested for the separation of isoprene/pentane mixture. The maximum selectivities for isoprene over pentane were found to be 53 and 83 for AgClO4-CA and AgBF4-CA membranes. respectively. Fr-IR and UV-Vis studies show that the silver ions in the membranes are coordinated by the carbonyl groups of CA. Reversible diene coordination to silver ions was observed by FT-IR and UV-Vis studies. Treatment of the AgBF4-CA membrane placed in a gas cell with isoprene produces an isoprene-coordinated membrane in which the coordinated isoprene is easily replaced by other diene compounds such as trans-piperylene and 1,3-butadiene. (C) 2004 Elsevier B.V. All rights reserved.