The oxygenation of water by a pure oxygen stream and the removal of dissolved oxygen from water by a nitrogen sweep gas were studied in a transverse flow module made with 1058 hydrophilic polysulfone hollow fibers. In the range of water flow rates of 0.3-3 l h(-1) the overall mass-transfer coefficient ranges from (0.52 to 1.8) x 10(-6) m s(-1), and from (0.77 to 3.7) x 10(-6) m s(-1) for water flow outside and inside the hollow fibers respectively. For water flow outside the fibers the main resistance to oxygen transfer is in the liquid-phase boundary layer, whereas for water flow inside the fibers the main resistance is in the membrane. The liquid-phase mass-transfer coefficient is proportional to the water flow rate raised to powers ranging from 0.63 to 0.67. The mass transfer coefficient of the membrane phase at 291 K ranges from (3.6 to 4.7) x 10(-6) m s(-1), which is three orders of magnitude lower than that for hydrophobic polypropylene hollow fibers. Nevertheless, polysulfone hollow fibers can be successfully used at low water flow rates to reduce the dissolved oxygen level in feed water from 8-9 mg l(-1) to less than 0.04 mg l(-1) or to achieve almost complete saturation of water with pure oxygen.