Gas transport properties of O-2 in poly(perfluoro sulfonic acid) (Nation 112) and disulfonated poly(arylene ether sulfone) were compared. While O-2 permeabilities were lower in poly(arylene ether sulfone) when compared to poly(perfluoro sulfonic acid), the O-2 gas sorption in the former was higher than in Nafion 112. Sorption measurements have been carried out both under non-humidified and humidified conditions. Under non-humidified conditions, the enthalpies of sorption (Delta H-s) were calculated as a function of pressure for both Nation 112 and disulfonated poly(arylene ether sulfone). The vertical bar Delta H-s vertical bar ranged from 6.58 to 7.30 kJ/mol for the disulfonated poly(arylene ether sulfone) and 4.94 to 5.36 kJ/mol for Nation 112 sample. O2 sorption was also measured as a function of relative humidity and temperature at a normal proton exchange membrane (PEM) fuel cell operating pressure of 1 atm. The disulfonated poly(arylene ether sulfone) was found to have a higher percent O2 mass uptake than Nafion 112. While under dry conditions, O2 sorption followed Henry's law, in the presence of humidity, O2 sorption followed Type IV behavior. (C) 2007 Elsevier B.V. All rights reserved.