The partial hydrogenation of 1,5-cyclooctadiene was performed in a pore-flow-through membrane reactor at 50 degrees C and 1 MPa hydrogen pressure with single capillary membranes and a bundle of 27 capillaries for scale-up studies. A preparation method for the catalytically active membranes and their characterization are described. Compared to millimeter-sized spherical catalyst pellets in fixed-bed or slurry reactors, mass transfer limitations could be successfully reduced in the membrane reactor. The conversion rate of 1,5-cyclooctadiene in the membrane reactor was mainly affected by the Pd amount and the mass flow rate of the reaction mixture through the membrane. Under optimum conditions a selectivity of 95% for cyclooctene at complete conversion of 1,5-cyclooctadiene was obtained. This value corresponds to the selectivity obtained with a,powder catalyst with a particle size of approx. 25 mu m in a slurry reactor for which microkinetic control of the reaction is expected to result in the highest possible selectivity. (C) 2007 American Institute of Chemical Engineers.