Thin film composite membranes based on sulfonated poly(phenylene oxide) (SPPO) as the top selective layer and PES ultrafiltration membrane as the support layer, were prepared. The effect of the support membrane pore size distribution, and the effect of exchanging the proton of the sulfonic acid groups with mono-, di- and trivalent cations on the gas transport properties of the TFC membranes were studied. It was found that the resistance to gas permeation imposed by the PES support layer increased significantly upon exposure to the solvent in the coating solution. The differences in the performance of the TFC membranes having different metal cation replacing the proton of the sulfonic acid groups were explained in terms of the metal cations' electronegativities and sizes. Very high selectivity associated with reasonable permeance was obtained when the SPPO was in the Al3+-form.