Nafion (R)-PBI composites were prepared by diffusing synthesized PBI from solution phase into Nafion (R) membranes, using different concenrations and drying temperatures. In some cases, Nafion (R) was treated with diethyl amine to screen the -SO3H groups and thereby avoid the strong acid-base interactions between the polymers during diffusion. The presence of PBI in the membranes was characterized with FT-IR spectroscopy. The performance of the membranes was studied by in-plane conductivity and methanol permeability. The performance ratio (the ratio between conductivity and methanol permeability compared to Nafion (R) increased by up to 50% for the composite membranes compared to Nafion (R). (C) 2007 Elsevier B.V. All rights reserved.