A submicrometer optical fiber oxygen sensor has been fabricated, based on the fluorescence quenching of tris-(1,10-phenanthroline)ruthenium(II) chloride in the presence of oxygen or dissolved oxygen. The Ru compound has been incorporated into acrylamide polymer that is attached covalently to a silanized optical fiber tip surface by photoinitiated polymerization. Leaching of the sensing reagent from the polymer host matrix has been minimized by the optimization of the ratio between the acrylamide monomer and the cross-linker, N,N-methylenebisacrylamide. The sensor is fully reversible and highly reproducible. A standard deviation of approximately 2% for 10 consecutive fluorescence measurements has been observed for several oxygen concentrations. The sample volume required for measurements is 100 fL. An absolute detection limit of 1 x 10(-17) mol is achieved. This is an improvement by a factor of 10(6) as compared to other existing optical fiber oxygen sensors.