Free volume, O(2) permeability, and uniaxial compression storage modulus of biphenol-based sulfonated poly(arylene ether sulfone) (BPSH) with an ion exchange capacity (IEC) of 1.60 mequiv./g were studied as a function of relative humidity at room temperature. The free volume hole size was quantified using the positron annihilation lifetime (PAL) technique, whereas the uniaxial compression storage modulus was measured by dynamic nanoindentation. Favorable formation of positronium (Ps) due to the anti-inhibition effect of -SO(2)- allowed for the application of the PAL technique to the study of free volume in BPSH. It was found that as the polymer is hydrated the free volume hole size (V(FV.Ps)) and O(2) permeability (PO(2)) first decrease and then increase. A semi-logarithmic plot of PO(2) versus 1/V(FV.Ps) for BPSH at different relative humidities showed a good correlation similar to that for different polymers such as polystyrene and polycarbonate in the dry state. Tendency of PO(2) deviating from the correlation in highly hydrated BPSH suggests an effect of polymer softening on O(2) permeation in addition to the dominant effect of the free volume. (C) 2010 Elsevier B.V. All rights reserved.