High molecular weight sulfonated polyphenylene oxide in protonated form (HSPPO) was modified by substitution of the proton in sulfonic groups by various metal cations, including Na+, Mg2+ and Al3+. Physico-chemical properties along with gas transport properties of the modified polymer - sulfonated polyphenylene oxide in metal form (MeSPPO), were determined and compared with the properties of the unmodified polymer. For a given degree of sulfonation (DS), MeSPPO is more hydrophilic than HSPPO. After substitution of the proton by a metal cation sulfonic groups become more thermally stable. Despite a greater density, MeSPPO is more permeable to gases than HSPPO. Permeability ratios for MeSPPO of higher DS are the same or greater than for the corresponding HSPPO. Consequently, substitution of the proton in sulfonic groups by metal cations improves gas transport properties of MeSPPO.