Desorption resistance taking place between a membrane surface and a permeate vapor phase, which had not accounted for an overall mass transfer resistance in pervaporation, was studied. The resistance-in-series concept and Flory-Huggins thermodynamics were used to establish model equations for evaluating the desorption resistance in the permeation of a single component. In order to exclude any possible concentration polarization of permeants occurring in feed adjacent to a membrane surface, the permeations of pure water through polyether imide membranes with various thicknesses were observed at different permeate pressures. From the permeation data of pure water through the membranes with help of the model equations, both the permeability coefficient based on a general flux equation expressed in terms of the chemical potential driving force and the desorption resistance were determined quantitatively. According to the model equations, the desorption resistance could be affected by two factors: membrane thickness and permeate pressure. The magnitude of the desorption resistance was dependent mainly on permeate pressure, and the importance of the resistance relative to diffusion resistance in the membrane for the overall process became more significant with decreasing membrane thickness at a given permeate pressure. As the membrane thickness decreased and/or the permeate pressure increased, the desorption resistance was observed to be more significant, causing higher chemical activity and a higher concentration of the permeant at the downstream interface of the membrane. In some cases, the desorption resistance was predominant over the diffusion resistance in very thin membrane thicknesses. This study seeks to emphasize the importance of the desorption resistance on the transport of components at small membrane thicknesses or high permeate pressure.