The propylene solubilities were measured in solid polymer electrolyte films comprising AgBF4 or AgCF3SO3 dissolved in poly(2-ethyl 2-oxazoline) (POZ) or poly(N-vinylpyrrolidone) (PVP). The propylene solubility for 1:1 POZ:AgBF4 membrane is almost 25-fold higher than that for pure POZ and further is nearly 2-fold higher than that for 1:1 POZ:AgCF3SO3 membrane. The solubility difference between AgBF4 and AgCF3SO3 polymer electrolytes was interpreted in terms of the anion size and the lattice energy of salts, the interaction intensity of metal ion with polymer chains, and the bond length between the silver ion and the closest anion atom calculated by the ab initio method. As a result, it was found that the solubility was better correlated with the bond length than other parameters.