The mass-transfer mechanisms of water vapor ill hollow-fiber membrane modules used to remove gases from water were studies experimentally ann theoretically. Hollow-fiber modules for degassing are widely used in various industries, including ultrapure water production. Although these modules perform extremely well, a general principle for the design has not been established. Recently very highly efficient asymmetric hollow-fiber modules I were developed, that could reduce rite dissolved oxygen to less than I mu g/L (1 ppb). From our studies Of the modules, it was found that the mass transfer. of water vapor plays nr? extremely important role in the flow in the lumen sine of the hollow fibers. The establishment of a theoretical design method for degassing modules can be expected front our studies.