Diffusion coefficients of H+ and Na+ ions in sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) intermolecularly crosslinked with poly(2-butylaniline) (PBA) or poly-[2,2'-(m-phenylene)- 5,5'-bibenzimidazole] (PBI) were evaluated by conductivity and H+/Na+ ion exchange measurements. Ion exchange capacity and transport properties of the PPO membranes with a degree of sulfonation higher than 30% are comparable to those of Nafion. The observed deterioration of ion exchange capacity and ion transport with increasing content of the cross-linking agent is related to the partial occupation of the fixed sulfo groups by the tertiary nitrogen groups present in PBA and PBI. (C) 2003 The Electrochemical Society.