We have already described some properties and some different mathematical aspects of the kinetics of membrane-bound enzymes. Here, we show the mechanism of an interfacial enzymatic reaction resulting in facilitated metabolite diffusion through a membrane. Several authors have reported that carbonic anhydrase facilitates CO2 transport in a liquid phase. The same effect is obtainable in the transfer of this gas through membrane-liquid interfaces giving a diffusion - reaction using bicarbonate as a transporter, especially when these interfaces become rate-controlling by concentration polarization. This is an example of the role of enzymes in lessening such an interface barrier. The membranes used in our experiments were prepared by covering hydrophobic sheets (Silastic) on one or both faces with chemically combined, reticulated carbonic anhydrase. Neither liquid water nor ions (such as bicarbonate) can cross the hydrophobic film, but it is well known that gaseous CO2 penetrates it quite easily.