Water vapor permeation through a silicone rubber-PSF (polysulfone) resistance composite membrane was studied. The resistance of the asymmetric composite membrane to the permeation of nitrogen, hydrogen, and water vapor is discussed on the basis of a serial resistance model. Compared with nitrogen and hydrogen, the resistance to water vapor is not attributable to the skin layer but is mainly due to the support layer of high water vapor permeability. The water vapor permeability decreases considerably with the increase of support layer resistance; the effect was greater when a relatively high vacuum was applied to the permeate gas side of the membrane. This result was also verified by the experiments of gas and vapor permeation through the composite membrane. In addition, the effects of the membrane structure on the water vapor permeability, as well as on the resistance of the support layer, were also investigated. The membrane dehumidification process was used because it increased the porosity, the pore size of the support layer, and the thickness of the skin layer.