Abstract: An apparatus for electrochemically determining the partial oxygen pressure pO.sub.2 in a liquid measuring medium includes a measuring sensor having a working electrode and a counter-electrode. The electrodes are manufactured in thin-film technology and are arranged on a substrate in a measuring channel, to which the measuring medium and a rinse medium can be supplied in alternation. The measuring channel connects a device which controls the flow rate of the measuring medium to a lumen of a double lumen catheter. The rinse medium is suppliable to a second lumen of the double lumen catheter with reversible flow direction. Both lumens have a common orifice which can be introduced into the measuring medium. So-called poisoning of the electrodes in this arrangement is suppressed because the electrodes are flooded in alternation by the measuring medium in a measuring phase and by the rinse medium in a regeneration phase.