The transport of water vapor through a Nafion membrane includes absorption of water at one membrane/gas diffusion layer interface, water transport in the membrane, and desorption of water at another interface. Based on the structure of the membrane, a model for the water transport through the membrane is presented. It was assumed that the mass-transfer coefficients for the absorption and desorption of water and the water diffusion coefficient were dependent on the volume fraction of water <mml:math display="inline"><mml:mrow><mml:mo>(</mml:mo><mml:msub><mml:mi>f</mml:mi><mml:mi mathvariant="normal">V</mml:mi></mml:msub><mml:mo>)</mml:mo></mml:mrow></mml:math>(fV) in the membrane. These parameters were determined from steady-state water transport flux through the membranes at different water activity gradients. The results show that the mass-transfer coefficient for the absorption of water ( <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mi mathvariant="normal">a</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>3.53</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>5</mml:mn></mml:mrow></mml:msup><mml:mi>f</mml:mi><mml:mi mathvariant="normal">v</mml:mi><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">m</mml:mi><mml:mo>∕</mml:mo><mml:mi mathvariant="normal">s</mml:mi></mml:mrow></mml:math>ka=3.53×10−5fvm∕s , <mml:math display="inline"><mml:mrow><mml:mn>353</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">K</mml:mi></mml:mrow></mml:math>353K ) is much lower than that for the desorption of water ( <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>k</mml:mi><mml:mi mathvariant="normal">d</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>1.42</mml:mn><mml:mo>×</mml:mo><mml:msup><mml:mrow><mml:mn>10</mml:mn></mml:mrow><mml:mrow><mml:mo>−</mml:mo><mml:mn>4</mml:mn></mml:mrow></mml:msup><mml:mi>f</mml:mi><mml:mi mathvariant="normal">v</mml:mi><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">m</mml:mi><mml:mo>∕</mml:mo><mml:mi mathvariant="normal">s</mml:mi></mml:mrow></mml:math>kd=1.42×10−4fvm∕s , <mml:math display="inline"><mml:mrow><mml:mn>353</mml:mn><mml:mspace width="0.3em"></mml:mspace><mml:mi mathvariant="normal">K</mml:mi></mml:mrow></mml:math>353K ). The parameters <mml:math display="inline"><mml:msub><mml:mi>k</mml:mi><mml:mi mathvariant="normal">a</mml:mi></mml:msub></mml:math>ka and <mml:math display="inline"><mml:msub><mml:mi>k</mml:mi><mml:mi mathvariant="normal">d</mml:mi></mml:msub></mml:math>kd describe the nonequilibrium water uptake of the membrane on operating conditions. Using these obtained parameters, the simulation results agree well with experimental data under many different conditions, including thickness of the membrane, water vapor/liquid water, temperature, and flow rate.