Air permeability and water vapor permeability of naphion membranes were measured using specially built instruments. Air permeability was almost zero. Water vapor permeability was zero during an incubation period. After the incubation period, the permeability became large and then gradually decreased with time. The behavior has been attributed to the contributions of chemical and mechanical forces to the net flux through the membrane. A model consistent with these results has been presented to explain absorption and transport of water vapor through naphion membranes.