When poly(N-vinyl pyrrolidone-co-vinyl acetate) (PVP-co-PVAc) containing amide and ester groups were complexed with silver salts to form silver polymer electrolyte membranes, their separation performance of propylene/propane mixtures showed the high selectivity of propylene over propane of 55 and the high mixed gas permeance of 12 GPU (1 GPU = 1.0 X 10(-6) cm(3)(STP) cm(-2) s(-1) cmHg(-1)). The separation performance strongly depends on the composition of the copolymer: the higher concentration of PVP in the copolymer, the better separation performance was achieved. These results suggest that the amide group is more effective in facilitated propylene transport than the ester group, primarily due to the stronger interaction of the silver ions with the amide than the ester oxygens, as demonstrated by FT-IR and FT-Raman spectroscopies. In-situ FT-IR spectra upon propylene sorption also demonstrate that the interaction strength of the silver ions with the ligands is arranged: amide > C=C > ester. (c) 2007 Wiley Periodicals, Inc.