This paper introduced a one-dimensional analytical model to quantitatively examine the humidifying capacity of a Nafion (TM) membrane humidifier. Water permeability for the Nafion (TM) membrane was calculated, depending on the thickness of the Nafion (TM) membrane. Relative humidity for a carrier gas passed through the Nafion (TM) membrane humidifier was simulated at various gas flow rates, which showed good agreement with experimental data. Simulations were also conducted to predict humidity levels as a function of length and height of the gas flow channel. This analytical model can be used in future work to properly design Nafion (TM) membrane humidifiers for use in PEM fuel cell systems. (C) 2008 Elsevier B.V. All rights reserved.