Sensing the chemical environment in confined spaces that characterize localized corrosion requires special instrumentation beyond the capability of classical electrochemical instrumentation. We describe a fiber-optic oxygen sensor based on fluorescence quenching of a ruthenium phenanthroline complex that is small, fast, and rugged enough to be used in the field. The sensitivity of the dual-fiber probe is 0.9 Torr and 0.01 mg/L oxygen in the gas and aqueous phases, respectively. The sensor utilizes a polydimethylsiloxane matrix for the dye that will facilitate further miniaturization for corrosion studies. (C) 2008 The Electrochemical Society.