Pyrrolidinium-based ionic liquids with a basic anion were shown to be highly effective acetylene transport carriers for the selective separation of acetylene in ethylene through membranes. The selectivity of acetylene over ethylene varied significantly depending on the type of the anion of the pyrrolidinium-based ionic liquids. The highest selectivity of 23 was attained when N,N-dimethylpyrrolidinium methylphosphite ([DMPyrr][MeHPO(3)]) was used as the transport carrier. Computational calculation and (1)H NMR results on the interactions of acetylene and ethylene with [DMPyri][MeHPO(3)] clearly indicate that the higher selectivity of acetylene over ethylene is originated from the specific acid-base interaction between methylphosphite anion, [MeHPO(3)](-) and the acidic hydrogen atom(s) of acetylene. (C) 2010 Elsevier B.V. All rights reserved.