Interactions of cation/anion and cation/polymer in poly(N-vinyl pyrrolidone) (PVP):silver triflate (AgCF3SO3) electrolytes with different weight-average molecular weights (M-w's) of 1 x 10(6) (1 M), 3.6 x 10(5) (360 K), 4 x 10(4) (40 K), and 1 x 10(4) (10 K) have been studied with IR and Raman spectroscopies. According to the change of the C=O peak, coordination of silver ions by C=O in a low M-w (10 or 40 K) PVP matrix tend to be always thermodynamically favorable than high M-w (1 M or 360 K) PVP, demonstrating that the polymer matrix of low M-w dissolves silver salts more effectively. In addition, silver cations interact with both larger SO3- and smaller CF3 to form ion pairs, and the former interaction is stronger than the latter in a monomer or low M-w polymer matrix (40 K, 10 K), as demonstrated by theoretical ab initio calculation or experimental spectroscopy, respectively. However, CF3 interacts more favorably with silver cation than SO3- in high M-w (1 M and 360 K) PVP, which is ascribed to the steric effect of the bulky SO3- anion by highly entangled polymer chains. Despite the superior dissolving property of the low M-w polymer matrix, the membranes consisting of low M-w PVP and AgCF3SO3 exhibited poor separation performance for propylene/propane mixtures in comparison with those of high M-w, presumably because of the poor mechanical property for membrane formation in low M-w PVP.