The effect of membrane type - structure and wettability - on the operation of two-phase, aqueous-organic, membrane bioreactors has been studied. The influence of surfactants on membrane wettability is reported. A simple, but highly sensitive, technique for the measurement of breakthrough pressures is described. Experimental measurements of the variation in break through pressures as the concentration of tenside in the system was changed are reported. On the basis of the results from these measurements it is concluded that : (i) hydrophilic and, highly retentive, amphiphilic ultrafiltration membranes may be used to operate two-phase bioreactors, (ii) amphiphilic microfiltration membranes should never be used in such reactors and (iii) PTFE membranes would always be a poor choice for use in such devices because they always have a low breakthrough resistance in two-liquid systems - breakthrough pressures as low as 100 mbar were observed for the system ethyl laurate-water-PTFE, which contains no surface-active component. It is shown that these results are in general agreement with rules of thumb for the selection of membranes, presented earlier. The influence of membrane history on its wetting behavior due to effects such as polymer surface restructuring - is highlighted. The limits on the utility of simple breakthrough pressure tests in determining suitable membranes, for use in two-phase bioreactors, owing to possible complications resulting from the exact mechanism of enzyme action is pointed out.