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. Three different flow configurations have been investigated: flow inside hollow fibres, flow outside and across woven hollow fibre bundles and flow in thin channels. For flow inside the fibres, the results obtained are in close agreement with analogous theoretical correlations. The results for flow across bundles of woven hollow fibres may be compared to analogous results for flow in cross flow heat exchangers. For flow in thin channels, the results obtained here are in agreement with the analogous theoretical heat transfer correlation developed by Leveque. However, at Graetz numbers less than 10, the results deviate from the predictions of Leveque. This deviation cannot be predicted by extensions to the Leveque solution or from the more rigorous analysis by Graetz. However it is possible to predict the observed deviation by considering slight polydispersity in the thickness of the thin liquid flow channels.