Cu(Ⅰ)-containing room temperature ionic liquids (Cu-RTILs),prepared from CuCl and alkylmethylimidazolium alkylphosphites([RMIM][RHPO3]),were found to reversibly and selectively interact with propyne over propylene.Cu-RTILs exhibited much higher propyne absorption capacity and ideal propyne/propylene selectivity than pure [RMIM][RHPO3].Fast atom bombardment (FAB)-mass spectral and computational results strongly imply that Cu-RTILs contain a highly stable alkylphosphite-eoordinated anionic Cu(1) species such as CuCl(RHPO3)-or Cu(RHPO3)2-.Computational studies on the optimized structures demonstrate that the preferential absorption of propyne over propylene in a Cu-RTIL is originated from the difference in the interaction mode between the coordinated phosphite ligand and propyne or propylene.Complexation between the Cu atom of Cu-RTIL with propylene and propyne is not observed.