The growth of several micro-organism and the formation of primary metabolites are impaired by the high concentrations of the products. Their removal during the cultivation is desirable. This can be realised by different methods. Reactive-Membrane-Extraction could be useful if not ethanol but the ester is the desired product. Ethanol should be extracted from the aqueous phase into an octane phase in a hollow fiber module and converted in to the phase with caprylic (octanoic) acid in ethylcaproate.