The invention relates to a method for determining the transmembrane pressureduring anextracorporeal blood treatment in which blood flows at a defined blood flowrate throughan arterial blood conduit 6 of an extracorporeal blood circuit 5A into theinlet of a firstchamber 3 of a dialyzer 1, which is divided by a semipermeable membrane 2 intothe firstchamber and a second chamber 4, and flows through a venous blood conduit 7from theoutlet of the first chamber 3 of the dialyzer 1, while dialysis liquid flowsthrough adialysis liquid supply conduit 10 into the inlet of the second chamber of thedialyzer andflows through a dialaysis liquid discharge conduit 11 from the outlet of thesecondchamber of the dialyzer. The method according to the invention and the deviceaccordingto the invention for determining the transmembrane pressure are such that thepressure onthe blood side and on the dialysis liquid side of the dialyzer is measuredwith relativelylittle technical outlay, specifically with fewer than four pressure sensors20, 21, 22, and apreliminary uncorrected value is calculated for the transmembrane pressure andisthereafter corrected by a correction variable that is dependent on a variablecorrelatingwith the viscosity of the blood.
