Preparation of solution-processed perfluorosulfonate ionomer membranes containing both small alkali metal and large alkylammonium counterions has been shown to have a profound impact on the mechanical and transport properties of the resulting acidified ionomer. The use of mixed counterions is shown to be an effective means of tailoring the thermomechanical properties of the membrane as evidenced by compositionally dependent relaxations in dynamic mechanical analysis. In agreement with our recent assignments, the alpha-relaxation is found to be systematically dependent on the strength of electrostatic interactions, whereas the T-g of Nafion (R) (i.e., the beta-relaxation) is susceptible to plasticization. Investigations of ionic aggregation using solid-state Na-23 NMR and small-angle X-ray scattering provided information suggesting the presence of mixed aggregates containing populations of both sodium and tetrabutylammonium ions. In contrast to the general perception that proton conductivity tracks with water content, membranes prepared at a 50:50 sodium/tetrabutylammonium counterion composition, followed by conversion to the H+-form, showed a minimum in water content yet relatively high proton conductivity. This behavior suggests that specific interactions during processing affect the organization of the ionic domains and yield persistent structures that can significantly influence membrane transport properties. (c) 2006 Wiley Periodicals, Inc.