The Membrane Bioreactor for Biotransformations (MBB) is based on the aqueous/organic two-phase system, and uses a tubular silicone rubber membrane to separate the two liquid phases. This avoids the key problem associated with direct contact two-phase processes, specifically, product emulsification. The baker＇s yeast mediated reduction of geraniol to citronellol was used as a model biotransformation to demonstrate MBB operation. Values for the overall mass transfer coefficient were determined for geraniol, (2.0 x 10(-5) mS-?), and for citronellol, (2.1 x 10(-5) ms(-1)) diffusion across the silicone rubber membrane. Using these values, and the specific activity of the biocatalyst (5 nmols(-1)g biomass(-1)), a suitable membrane surface area : biomass ratio was determined as 2.4 x 10(-3) m(2)g biomass(-1). The bioreactor was operated at this surface area : biomass ratio and achieved a product accumulation rate 90-95% that of a conventional direct contact two-phase system. The slight reduction in product accumulation rate was shown not to be due to mass transfer limitations with respect to reactant delivery or product extraction.