A hollow-fibre permeator containing silicone rubber membranes has been used for the removal of CO2 from a breathing gas mixture. The effect of permeate gas absorption with water in the shell side of the permeator has been investigated and compared with conventional systems with and without permeate gas purge. Experimental results compare favourably with theoretical predictions. It has been shown that the use of an aqueous solution as an absorbing medium in the permeate side of the permeator can greatly improve CO2 separation from the gas mixture and reduce the need to maintain a high pressure ratio across the membrane. The improved CO2 selectivity is found to be caused mainly by the liquid film which, however, creates film resistance, resulting in much reduced CO2 permeation flux. The CO2 permeation flux can be improved using an alkaline solution instead of neutral water.