The mechanism of gas permeation in sulfonated polymers such as Nafion and sulfonated polyethersulfone (SPES) for polymer electrolyte fuel cells has been investigated from the viewpoint of free volume. Free volume was quantified using the positron annihilation lifetime (PAL) technique and O2 permeability was measured as a function of temperature or ion exchange capacity (IEC). Good linear correlations between the logarithm of the permeabilities and reciprocal free volume indicate that gas permeation in the sulfonated polymers depends strongly on the free volume. Comparison of Nafion with SPES and other relatively flexible chain polymers showed that the stiff chains of the perfluoroethylene backbone in Nafion make the O2 permeability much smaller than the corresponding flexible chain polymer with a similar free volume size. The O2 permeability in the K+-form of Nafion is systematically smaller than the H+-form, showing that the chain stiffness is enhanced by the ion exchange.