Catalytically active membranes can be applied for three-phase reactions (liquid, gas and solid catalyst) and have advantages over conventional particle catalysts. Catalytically active components are deposited in the thin fine-porous membrane layer of an asymmetrical ceramic membrane. One reactant is dissolved in the liquid and diffuses through the porous structure of the membrane to the active inner surface, the other reactant is fed through the support to the catalytic layer from the other side of the membrane. Thereby, an effective contact between the two reactants and the solid catalyst is established. Under these conditions catalytically active membranes can typically be applied for hydrogenation or oxidation processes. Alternatively, if pore diffusion needs to be eliminated the dissolved reactants can be pumped through an asymmetric ceramic membrane or just a ceramic support coated with catalytically active metals. Thus, a very short contact time can be achieved. The following paper compares these two concepts-the catalytic diffuser and the forced through flow concept-and discusses their application for hydrogenation processes, like the catalytic nitrate/nitrite reduction in water and the dechlorination of chlorinated hydrocarbons. The developed catalytic membranes are not limited to these reactions, but are applicable for a number of multiphase reactions. A preparation method in order to make ceramic membranes catalytically active and different characterization methods will be described. (C) 2003 Elsevier B.V. All rights reserved.