We estimated the energy barriers of proton transfers in the systems of (CF3SO3/H/SO3CF3)(-) and (CF3SO3/H/H2O/SO3CF3)(-) as models of a water-swollen Nafion membrane by an ab initio density functional quantum calculation method with consideration of the hydration effect. As a result, the proton transfer between the SO3- sites, which is accompanied by one water molecule, was found to be one of the proton-transfer mechanisms in the water-swollen Nafion membrane; that is, the surface diffusion mechanism was found to be important for the proton transfer in that membrane. (C) 2004 Wiley Periodicals, Inc.