Moisture permeation tests were conducted to measure the total moisture transfer resistance including the moisture transfer resistance through membrane and the boundary layer resistances on both sides of the membrane. Special methods were employed to determine the two boundary layer resistances, with the membrane resistance obtained by subtracting these two resistances from the total resistance. The membrane microstructures were observed using a scanning electron microscope. Tests were done on two porous membranes including the PES (poly ether sulfone) and cellulose membranes. The moisture transfer process through each membrane was modeled using a pore flow model, in which the pore sizes were assumed to obey the standard normal distribution and the transmembrane permeation was considered to include the Knudsen and molecular diffusions and the Poisseuille flow. The effects of membrane microstructure on transmembrane permeation were analyzed based on the model, with the moisture permeation test data and SEM observations as a basis. The results show that the membrane resistance decreases with increasing mean pore radius but changes little with the dimensionless standard deviation, further, the membrane resistance is in inverse proportion to the surface porosity to pore tortuosity ratio. (C) 2011 Elsevier B.V. All rights reserved.