On-line plasma detoxification requires two circulation: the separation of the plasma from the patients blood, in a first circulation, and a plasma fractionation, in a second circulation, before the detoxified plasma returns back to the patient. These processes can be realised in one single step by use of a duofilter. The duofilter is a modular construction integrating two different hollow fibre filters in one common module housing. In this paper the liquid flow and the pressure distribution in the duofilter module system is calculated based, on the one hand, on the laws of hydrodynamics, and on the other hand, on the finite volume method, where each part in the duofilter is scrutinized separately and supplemented with a computational simulation program for liquid flow. The data from such calculations are compared with experimental investigations. The results of both, calculation methods and the experimental technique, demonstrate correspondence. On this basis the module equipment can be optimized with respect to its performance.