Dissolved oxygen is one of the major contaminants that have to be removed in the production of ultrapure water. The removal of dissolved oxygen from ultrapure water employing microporous hydrophobic membranes has been studied. The membranes have a negligible resistance to the passage of oxygen and provide a larger surface area per unit volume for gas-liquid contact. A study of the mass transfer in the membrane gas-liquid contactor showed that the resistance in the liquid film adjacent to the membrane phase controlled the rate of oxygen removal. The Leveque equation which describes adequately the observed overall mass transfer coefficients, fails to predict satisfactorily the dissolved oxygen concentrations in the parts per billion range. The experimental results also indicate that the membrane modules were capable of reducing the dissolved oxygen content in water to a level of around 8 ppb.