The high molecular weight polyphenylene oxide (PPO) was sulfonated to different ion exchange capacity (IEC) values using chlorosulfonic acid. The physico-chemical properties along with the gas transport properties of the membranes prepared from sulfonated PPO (SPPO) were evaluated. Sulfonation of PPO results in a linear increase of density with the IEC value while the average d-spacing in polymer remains constant. Sulfonic groups attached to the aromatic ring in the PPO backbone are not thermally stable. On the other hand, when tested with CO2 at room temperature, the SPPO membranes maintained a constant permeability over the period of 60 days. An increase in IEC value of SPPO results in an increase in O-2/N-2 and CO2/CH4 ideal selectivities and a decrease in O-2 and CO2 permeabilities. The combination of permeability and ideal selectivity for both gas pairs places the SPPO membranes below the respective upper-bound lines for polymeric membranes. However, an increase in the IEC value brings the permeability versus ideal selectivity relationship closer to the upper-bound Line, especially for the O-2/N-2 gas pair.