Abstract: A device for diffusing substances between two fluids via semi-permeable diaphragms for dialysis apparatus in general and in particular as so-called artificial kidneys. The diaphragms are disposed in pairs between spacer plates and each plate includes an arrangement of flow-through ducts for the respective fluid connected to inlets and outlets respectively. A first fluid is arranged to be ducted between the diaphragms of each pair and a second fluid is arranged to be ducted outside the diaphragms, the diaphragms forming between them a network of ducts for the first fluid. In order to make it possible to prevent such duct networks having a well defined thickness and volume, the diaphragms are joined together by gluing, heat sealing, or the like within a pattern of spots, lines and/or small areas.