Abstract: The present invention relates to a method for combined administration of carbon monoxide (CO) to an ex-vivo fluid and monitoring of the carbon monoxide administration, said method comprising: (i) generating CO by reacting a CO releasing molecule (CORM) with a release triggering molecule; (ii) administering CO to an ex-vivo fluid by contacting the ex-vivo fluid with the CO generated in step (i) via a gas-permeable membrane; (iii) analyzing carbon monoxide and/or a carbon monoxide marker after administering in step (ii) CO to the ex-vivo fluid by complementary monitoring techniques; (iv) adjusting the CO administration based on the analysis of the carbon monoxide or the carbon monoxide marker carried out in step (iii), if necessary. It furthermore relates to an extracorporeal circuit system for use in the method of the invention.