A reflection mode fiber optic oxygen sensor that can operate at high temperatures for power plant applications is being developed. The sensor is based on the {sup 3}O{sub 2} quenching of the red emission from hexanuclear molybdenum chloride clusters. The luminescence of Mo{sub 6}Cl{sub 12} immobilized in a sol-gel matrix was measured as a function of heater temperature up to 200 C, in an inert environment. While the luminescence decreased with temperature, the integrated intensity at 200 C should be sufficient to enable detection of the luminescence in a fiber geometry. Previously we found that aging Mo{sub 6}Cl{sub 12} at temperatures above 250 C converts the canary yellow Mo{sub 6}Cl{sub 12} to a non-luminescent gray solid. Optical and thermal aging experiments show that the alkali metal salts of Mo{sub 6}Cl{sub 12} have higher thermal stabilities and remain luminescent after aging at 280 C.