Polyaniline/Nafion and polypyrrole/Nafion composite membranes, prepared by chemical polymerization, are studied by scanning electron microscopy, infrared and nuclear magnetic resonance spectroscopy. Differences in vanadium ion diffusion through the membranes and in the membranes' area specific resistance are linked to analytical observations that polyaniline and polypyrrole interact differently with Nafion. Polypyrrole, a weakly basic polymer, binds less strongly to the sulfonic acid groups of the Nafion membrane. Infrared spectroscopy results suggest that the hydrophobic polymer aggregates in the center of the Nafion channel rather than attaching to the hydrophilic walls containing sulfonic acid groups. This results in a drastically elevated membrane resistance and only slightly decreased vanadium ion diffusion compared to a Nafion membrane. Polyaniline, on the other hand, polymerizes along the sides of the Nafion pores and on the membrane surface, binding tightly to the sulfonic acid groups of Nafion, polyaniline's greater basicity possibly causing the difference in polymerization behavior. This leads to a more effective reduction in vanadium ion transport across the polyaniline/Nafion membranes and the increase in membrane resistance is less severe. The performance of selected polypyrrole/Nafion composite membranes is tested in a static vanadium redox cell. Increased coulombic efficiency, compared to a cell employing a pure Nafion membrane, further confirms the reduced vanadium ion transport through the composite membranes. (C) 2011 Elsevier B.V. All rights reserved.