The effect of ionic liquids on the formation of a partial positive charge on the surface of silver nanoparticle and its subsequent effect on facilitated olefin transport were investigated. Three different ionic liquids of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM+BF4-), 1-butyl-3-methylimidazolium triflate (BMIM+Tf-), and 1-butyl-3-methylimidazolium nitrate (BMIM+NO3-) were employed to control the positive charge density of the surface of silver nanoparticles. The positive charge density of the silver nanoparticles, as characterized by the binding energy of the silver atom, was in the following order: BMIM+BF4-/Ag >> BMIM+Tf-/Ag > BMIM+NO3-/Ag. This order was consistent with the tendency of ionic liquids to form free ions. The best separation performance for the propylene/propane mixtures was a mixed gas selectivity of 17 and a permeance of 7GPU through a composite membrane consisting of BMIM+BF4-/Ag. A better separation performance for olefin/paraffin mixtures was observed with a higher positive charge density of the silver nanoparticles. It was therefore concluded that facilitated olefin transport was a direct consequence of the surface positive charge of the silver nanoparticles induced by ionic liquids. (C) 2008 Published by Elsevier B.V.