The invention relates to an apparatus for extracorporeal blood treatment having a dialyzer which is divided into a first and a second chamber 29, 30 by means of a semipermeable membrane wherein the first chamber 29 is arranged in a dialysate path and the second chamber 30 can be connected to a patient's blood circulation by means of a blood supply line 32 and a blood discharge line 31, an inlet 20 for fresh dialysate, an outlet 36 for spent dialysate, a measuring device 37, wherein the measuring device 37 has at least one radiation source 1 for substantially monochromatic electromagnetic radiation. Thereby the measuring device 37 for determination of a luminescence of the spent dialysate flowing through the outlet 36 has at least one detector system (5) for detecting the intensity of the electromagnetic radiation generated by luminescence.
