A catheter system is suitable for the simultaneous, continuous, mutually unaffected measurement of the centrovenous oxygen saturation and the local concentration of injected indocyanine green. It has a centrovenous catheter with a fiber-optic lumen and a fiber-optic probe that can be inserted into the fiber-optic lumen for taking reflecto-oximetric measurements at a first wavelength of 660 nm, a second measuring wavelength of 805 nm and a reference wavelength of 880 nm. To prevent a longitudinal shift of the fiber-optic probe relative to the fiber-optic lumen, a connecting piece, securely connected to the fiber-optic probe, and a counterpart, securely connected to the catheter, are provided which can be joined together. The fiber-optic lumen which continues in the interior of the connecting piece can be cleaned when the connecting piece is attached to the counterpart. For this purpose, the connecting piece has a cleansing attachment for connecting a cleansing device. The cleaning takes place through the cleansing channel.