The duomodule integrates two kinds of serially connected hollow fiber filters in a common module housing. The inner compartments of both filters are interlocked by a stable connection in which a throttle valve is integrated. The connected outer compartments of the filters are filled with an incompressible fluid. At a liquid flow in the inner compartment (primary circulation), a secondary circulation is generated due to a pressure drop between the filters. Filtration from the inner compartment to the outer compartments is initiated in the first filter, whereas refiltration takes place in the common outer compartment to the inner compartment in the second filter as a consequence of a transmembrane pressure reversal. In this paper, different duomodule arrangements are used for the fractionation of an aqueous dextran solution mixture to detect the duomodule separation ability and efficiency and to optimize the module construction. The fractionation ability and efficiency are related to the membrane characteristics. The results of the fractionation experiments demonstrate the separation function of the equipment. In the refiltration step, the transport direction goes from the outer to the inner membrane surface; therefore, an internal concentration polarization in the membrane wall can change the permeability and the separation behavior of the refiltration membrane. Accordingly, only symmetrically structured membranes or outside active refiltration membranes realize a desirable high fractionation ability and efficiency. (C)2000 Elsevier Science B.V. All rights reserved.