Ferric stearate and ferric complexes were successfully deposited onto porous support membranes. Langmuir-Blodgett (LB) films of ferric stearate greatly reduced the permeabilities of carbon dioxide and nitrogen through the membrane. Above 65-degrees-C, a time-dependent change in the LB film leads to a permeability increase. Permeation through ferric stearate alone was mostly due to a Knudsen diffusion mechanism. LB films selective for carbon dioxide were made by co-depositing poly(2-vinylpyridine) with ferric stearate and with the ferric salt of poly(maleic anhydride 1-octadecene).