Hollow fibre modules containing hydrophobic membranes have been assessed for their capability in the removal of carbon dioxide from a simulated breathing gas mixture. The overall mass transfer coefficients of carbon dioxide have been obtained in the gas phase. A study of mass transfer in the membrane modules indicates that the overall mass transfer coefficients, KG, are dominated by the individual coefficients in liquid film, k(L) The correlation Sh=1.164(Re d(e)/Z)Sc-0.967(0.33) gives the best fit to the mass transfer data obtained in the gas phase. Finally, a method is outlined which enables a quick estimate of the design parameters of such systems.