The use of membranes to separate liquid/liquid mixtures downstream of multiphasic biocatalytic reactor has been examined. Hydrophilic and hydrophobic membranes were used to separate multiphasic organic solvent/water mixtures. It was found that one phase could be separated from the mixture as long as the breakthrough pressure, derived from the Laplace law, was not exceeded. The breakthrough pressure was found to be a function of the pore diameter and the interfacial tension. The flux through the membranes was found to be function of the volume fraction of water. Between a water volume fraction of 0.4 and 0.8, both hydrophilic and hydrophobic membranes were found to give high fluxes in the absence of surface active material. Yeast and Pseudomonas putida were used as examples of microbial material present in liquid/liquid mixtures downstream of a multiphasic biocatalytic reactor. The fluxes of both membranes decreased upon addition of the cells to the mixture. While the hydrophilic membranes were reversibly fouled and retained acceptable fluxes, in contrast, the hydrophobic membranes were fouled irreversibly. Subsequently, the hydrophobic membranes were modified and the flux through the hydrophobic modified membrane remained high and was a function of the viscosity and solvent volume fraction. Both hydrophilic and hydrophobic membranes could also be used at high pressure when both liquids permeated the membrane. In this case, the membranes acted as coalescence filters although at a considerably reduced flux compared to low-pressure applications.