Polyion complexes of arachidic acid and poly(ethyleneimine) were deposited onto Celgard 2400 microporous support membranes and the permeabilities of carbon dioxide and nitrogen through these composite membranes were measured at elevated temperatures. While nitrogen is relatively inert, carbon dioxide tends to have a high solubility in the hydrophilic polymer and the resulting difference in the permeabilities of carbon dioxide and nitrogen can be used to characterize solution-diffusion permeation. Room-temperature gas permeabilities were consistent with Knudsen diffusion through holes or defects. At elevated temperatures the permeabilities of carbon dioxide increased sharply while those of nitrogen increased only moderately indicating that the solution-diffusion mechanism became competitive with Knudsen diffusion. Room-temperature reflection and transmission IR spectra of films on separate substrates indicated that the alkyl tails of the fatty acid were oriented parallel with the surface normal and were packed hexagonally. At elevated temperatures, the tails and the carboxylate head groups disordered with increasing temperature. The disordering was correlated with the sharp increase in the carbon dioxide permeability.