Poly-perfluorosulfonic acid membranes were doped with cesium ions to several degrees. These, along with the H+- form membrane, were investigated in relation to methanol permeability as well as to hydrogen ion conductivity. While retaining considerable conductivity, the cesium doped membranes are highly impermeable to methanol. We found that methanol permeability in the membrane reduced by over one order of magnitude, owing to the presence of cesium ions. These findings are discussed on the ground of the alterations produced by cesium in membrane micro-structure. Also discussed is the potential implication of these results in the direct methanol fuel cell technology.