Abstract: Quenching luminescence of the tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) perchorate, immobilized in a silicone rubber by oxygen is shown to be an accurate and precise method for measuring oxygen concentration in solutions and in the gas phase. Quenching can be quantitated by either lifetime or intensity quenching measurements. Strong aqueous acids and bases, complexing agents, oxidants, and reductants do not penetrate the hydrophobic polymer and, therefore, do not affect the response. Gaseous interferents, such as H.sub.2 S, anesthesia gases (e.g. N.sub.2 O, Halothane), and fluorocarbons do not affect the response. Chlorine and especially SO.sub.2 cause strong, but reversible interference presumably because of electron transfer quenching. A system with a response time of less <0.2 s is disclosed, which is adequate for the monitoring of breathing subjects.