Membrane gas transfer can be compromised by water condensation during operation. Experiments reported here show that the feed gas is rapidly saturated with water, whose transport is controlled by resistances in the membrane and the gas phase. This gas then becomes supersaturated because of gas dissolution, which is controlled by transport in the liquid phase. The water in the supersaturated gas is not normally transported back into the liquid, but forms droplets on the gas side of the membrane. While this condensation cannot normally be avoided, module design can facilitate condensate removal. (C) 2003 Elsevier B.V. All rights reserved.