The base monomer, 1-vinyl imidazole was impregnated in a Nafion 112 membrane and polymerized to poly(1-vinylimidazole) (PVI) by UV irradiation in order to reduce the methanol permeability of the Nafion membrane when used in a direct methanol fuel cell. As the PVI content in the composite membrane was increased, the equilibrium water uptake decreased and the size of the hydrated ion clusters was reduced, as confirmed by small angle X-ray scattering analysis. The electrochemical properties of the membrane, such as its proton conductivity, methanol permeability, and electro-osmotic drag, were also affected by the equilibrium water uptake and hydrated pore size. Even a small amount of the base polymer incorporated into the membrane resulted in a significant effect on its proton conductivity and methanol permeability. The methanol transport induced by the electro-osmotic drag was evaluated and the methanol permeability and limiting current density data obtained in this study. Although the absolute number of the electro-osmotic drag was not determined, the trend of change is discussed in relation to the bulk-like water in the composite membranes. The novel composite membrane exhibited improved cell performance compared with a plain Nafion membrane due to its reduced methanol crossover rate. (c) 2005 The Electrochemical Society. All rights reserved.