The direct regioselective hydrogenation of alpha,beta-unsaturated aldehydes in aqueous multiphase catalysis was investigated in a capillary microreactor with channel diameters ranging from 500 to 1000 mu m. By connecting two mixers in line, a regular and stable alternating plug flow behavior could be generated, allowing accurate measurement of the plug geometry and hence the calculation of the specific surface area relevant for mass transport. These parameters are essential for reliable kinetic modeling of multiphase reaction systems containing microkinetic as well as transport terms.