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A pervaporative catalytic membrane reactor can afford chemical reaction and separation or concentration of the products in one step. The hydrogenation of acetophenone (AP) in diluted aqueous solution was investigated applying catalytically reactive polydimethylsiloxane (PDMS) and polyether-b-amide (PEBA) membranes in the pervaporative membrane reactor. A quadrupole mass spectrometer (QMS) was employed to follow online the hydrogenation reaction. Homogeneously distributed palladium clusters inside the polymeric membranes catalyze the hydrogenation of AP to 2-phenylethanol (PE) and the consecutive product ethyl benzene (EB) at low temperatures (30-70°C). The reaction depends strongly on the disposability of H2 inside the membrane. Only by means of the online QMS analyser the availability of stoichiometric or excess amounts of H 2 at the catalyst with adjusted parameters such as temperature, hydrogen pressure, solute concentrations and membrane permeability could be detected. It could be shown, that the hydrogen level by bubbling at ambient pressure was not sufficient, however, at 4 bar H 2 pressure and 50°C conversions up to 83% in 6h were obtained. Efforts to quantify the measured intensities of the QMS were not successful; however, the relation of the organic compounds and the presence of excess H 2 were detected precisely with this method.