In modem blood oxygenators, microporous membranes (flat sheet or hollow fibre) are used to separate the blood and gas phases. Blood flows on one side of the membrane while gas (usually oxygen or air) flows on the other side. Since the membranes used are hydrophobic (e.g., polypropylene, Teflon), the pores are gas filled resulting in a negligible membrane mass transfer resistance. The major resistance to gas transfer is due to the blood side mass transfer boundary layer. In this study mass transfer and friction factor correlations for commercially available hollow fibre and flat sheet blood oxygenators have been determined experimentally. Water was used as a substitute for blood. The diffusion of oxygen into and out of water has been studied. Two different flow configurations used commercially have been investigated: flow outside and across woven hollow fibre bundles and flow in thin channels. The results for flow across bundles of woven hollow fibres may be compared to analogous results for flow in cross flow heat exchangers. The results obtained here can be used to further optimize the design of blood oxygenators.