The influence of cation form and degree of sulfonation on free volume, as probed via positron annihilation lifetime spectroscopy (PALS), and water and salt transport properties was determined in a systematic series of directly copolymerized disulfonated poly(arylene ether sulfone) random copolymers. Polymer samples were studied in both the dry and hydrated states. PALS-based estimates of free volume in the dry polymers were compared with those estimated using density and the Bondi group contribution method, and PALS-based free volume data for hydrated polymers were correlated with water and salt transport properties. The transport properties depend strongly on free volume cavity size. Samples with larger free volume elements have higher water and salt solubility, diffusivity, and permeability and lower water/salt diffusivity and permeability selectivity. Sorption of water alters the characteristic free volume of the polymer matrix by two competing mechanisms: water molecules partially occupy the original free volume in the initially dry polymer, thereby reducing free volume cavity size, and water swells the polymer matrix therefore increasing the mean free volume size as a result of increased polymer chain plasticization. The importance of the second effect increases as water uptake and, thus, plasticization increases.