Polymer electrolyte membranes composed of poly(2-ethyl-2-oxazoline) (POZ) and AgBF4 are prepared for olefin/paraffin separations. The stability and effect of relative humidity on permeance and selectivity are investigated using propylene/propane mixed gases. The selectivity of propylene over propane remains high on the long time operation with a dry feed gas, indicating that membranes are effective For propylene/propane separation in the absence of water. As humidity of the feed gas increases, both propylene and propane permeances increase, which resulting in the slight decrease of selectivity.