Abstract: A catheter system for blood gas monitoring and the method of making same comprising a polarographic sensing means having a body, first and second conductors supported by the body and each respectively having an exposed conducting surface at a region of the body, electrolytic material supported by the body at said region in contact with the exposed surfaces at the first and second conductors, and a membrane supported by the body extending over the region of said body covering the electrolytic material. The membrane is pervious to oxygen in the fluid system and semi-pervious to water. The electrolytic material is anhydrous prior to use of the device and is activated by immersing in a aqueous solution just before use. The sensing means is at the end of a cable carrying the conductors which cable has its other end joined to the connector end of a terminal or transport unit.