A highly porous polyimide film with tunable pore size, porosity and thickness is synthesized and used as a matrix to construct a Nafion-infiltrated composite membrane. A very efficient way for an easy and complete infiltration of the proton-conducting polymer into this substrate is developed, which is usually a major problem for composite membranes. Due to the complete inertness to methanol and the very high mechanical strength of the polyimide matrix, the swelling of the composite membrane is greatly suppressed and the methanol crossover is also significantly reduced (80 times), where as while high proton conductivity (comparable with Nafion) and mechanical strength (4 times stronger than Nafion) is still maintained. This membrane demonstrates significantly improved cell performance compared with the Nafion membrane and is a promising candidate for use in direct methanol fuel cells. (C) 2009 Elsevier B.V. All rights reserved.