Permeabilities of dense films made from high molecular weight sulfonated polyphenylene oxide in protonated (HSPPO) and sodium (NaSPPO) forms were determined by constant pressure (CP) and constant volume (CV) techniques in appropriate testing systems. The comparison of permeability data of similar films determined by the two testing systems reveals considerable differences.. In general, for any film the CO2/CH4 permeability ratio obtained by the CP system is larger than that obtained by the CV system. In case of the O-2/N-2 permeability ratio the situation is opposite. The observed differences are attributed to the fact that the water vapor pressure in the laboratory might have influenced the tests performed in the CP system. In addition, the order and duration of gas permeation tests, in particular the duration of the test with CO2, also influences permeabilities determined by the CP system. The effects of the water vapor pressure and film hydrophilicity on the apparent gas permeabilities are discussed.