Oxygen quenching of the luminescence of the trls(4,7-dl-phenyP 1,10-phensnthrollne )ruthenlum( 11) perchlorate Immobilized In a silicone rubber Is shown to be an accurate and precise method for measuring oxygen concentrations In solutions and In the gas phase. Quenching can be quantitated by either lifetime or Intensity quenching measurements. Aqueous strong acids, bases, complexlng agents, oxidants, and reducíante do not penetrate the hydrophobic polymer and, therefore, do not affect the response. Gaseous Interference, such as H2S, anesthetic gases (e.g., N20, halothane), and fluorocarbons do not affect the response. Chlorine and especially S02 are strong, but fully reversible, Interferents. A system was developed with a response time of less than 0.2 s, which Is adequate for the monitoring of breathing subjects.