In order to monitor corrosion risk of radioactive-waste repository, a fiber-optic oxygen sensor for long-term use was proposed and its durability at high temperature was evaluated. The sensing mechanism is based on quenching of luminescence by oxygen. Many kinds of Ruthenium complexes which have different counter-anion and ligand were utilized as oxygen-sensitive dye. Several Rucomplexes were dispersed into silicone resin. Excitation is achieved with a blue LED (470 nm) and the emitted light at around 610 nm is collected by optical fiber. The sensing membranes showed high sensitivity and quick response to gaseous and dissolved oxygen. Comparison of sensitivity and durability of the Rucomplexes were performed. Degradation behaviors of the sensors in air at 150°C were investigated. Also short-term degradation test in hot water (80°C) was conducted. Screening of the sensors is underway to find the most stable sensor in such environment by this test. © The Electrochemical Society.