The highly selective dry complex membrane AgBF4–cellulose acetate (CA) was prepared and tested for the separation of ethylene/ethane and propylene/propane mixtures. The maximum selectivity for olefin over paraffin was found to be 280 for the ethylene/ethane mixture and 200 for the propylene/propane mixture. Solid-state interactions of AgBF4 with cellulose acetate (CA) and/or olefins have been investigated by using FT-IR, UV, and X-ray photoelectron spectroscopy (XPS). FT-IR and XPS studies clearly show that the silver ions are coordinated by carbonyl oxygen atoms among three different types of oxygen atoms present in CA—two in the acetate group and one in the ether linkage. Upon incorporation of AgBF4 into CA, the carbonyl stretching frequency of the free cellulose acetate at 1750 cm−1 shifts to a lower frequency by about 41 cm−1. The binding energy corresponding to a carbonyl oxygen atom in the O 1s XPS spectrum shifts to a more positive binding energy by the incorporation of AgBF4. Reversible olefin coordination to silver ions has been observed by FT-IR and UV studies. Treatment of the AgBF4–CA membrane placed in a gas cell with propylene produces a propylene-coordinated membrane in which coordinated propylene is easily replaced by other olefins such as 1,3-butadiene.