Graft copolymers (PAN-g-macPSSNa) were prepared by copolymerization of acrylonitrile with poly(sodium styrene sulfonate) macromonomers (macPSSNa). Macromonomers were prepared by stable free radical polymerization. Polymers containing different graft chain densities were solvent-cast to provide a series of membranes of varying ionic content. Membranes were protonated and characterized with respect to water content, ionic conductivity, and as a medium for the electrochemical reduction of oxygen. Membrane properties were compared to other solid polymer electrolytes containing poly(styrenesulfonic acid) graft chains. Compared to analogous membranes that contain the more hydrophobic polystyrene backbone, polyacrylonitrile-based membranes uptake more water for a given ion exchange capacity. This has the effect of lowering proton conductivity, due to dilution of ions; increasing the diffusion coefficient of oxygen; and decreasing the oxygen solubility in the membrane. Due to the offsetting effects of the increased diffusion coefficient and decreased solubility, the oxygen permeability through polystyrene- and polyacryonitrile-based membranes is similar for a given ion exchange capacity. (C) 2003 The Electrochemical Society.