A simple formula for water permeability of a Nafion hollow-fiber membrane has been determined. The result shows that water permeability of the membrane is proportional to the 1.5 power of the local partial pressure of water vapor in the gas phase. The water permeability was converted to a diffusion coefficient, which was found to attain a maximum value when the number of water molecules per sulfonate in the Nafion membrane is equal to 3.3. This agrees well with some reports in the literature.An effective mathematical model for a membrane process to remove water vapor from nitrogen gas was developed. Using the obtained permeability expression, the model simulations for the membrane module matched experimental data over a wide range of operating conditions. In most cases, the absolute errors between the predicted product relative humidity and the measured values are within +/-2%, and the relative errors are less than 15%. Mass transfer resistances along the membrane module at typical operating conditions were also analyzed. (C) 2003 Elsevier B.V. All rights reserved.