A generally accepted method for the determination of high oxygen permeabilities (Dk > 100 barrers) of water-immersed membranes is unavailable. We designed and developed a generally applicable method, together with simple equipment, to measure the oxygen permeability up to Dk similar to 800 barrers of highly oxygen permeable membranes in contact with water. A theory of the methodology is also presented, giving particular attention to the boundary layer effect and the edge effect. The practical applicability of our technique is demonstrated by preparing and using highly oxygen permeable water-logged membranes, such as polydimethylsiloxane and polysiloxane copolymers important for medicine. According to our measurements, the Dk's of polydimethylsiloxane, poly(dimethylsiloxane(0.80)-co-diethylsiloxane(0.20)), and poly(dimethylsiloxane(0.84)-co-diphenylsiloxane(0.16)) are 792 +/- 26, 505 +/- 10, and 249 +/- 10 barrers, respectively. Evidently, the oxygen permeabilities of polysiloxanes are strongly reduced by substituting the -OSiMe2- repeat unit with the structurally similar -OSiEt2- and -OSiPh2- repeats. (c) 2005 Wiley Periodicals, Inc.