Gas transfer performance is presented for one form of the Oxford membrane lung in which vortex mixing is induced in blood flow across a dimpled polypropylene membrane. Dimensional analysis has been used to define the parameters characterizing mass transfer in the device, and of three fluid mechanical parameters: Reynolds number based on peak pulsation velocity, Strouhal number, and ratio of mean to oscillatory flows, only the first has been found to affect mass transfer significantly in the ranges studied. For oxygenation, rated flows in excess of 5 l min-1 m-2 are measured. A new definition is presented for a rated flow for extracorporeal CO2 removal and values in excess of 1.2 l min-1 m-2 are obtained.