A fiber optic chem. sensor is described for the determination of oxygen concentration in water based on the dynamic luminescence quenching of a fluorophor by oxygen. [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) was as the oxygen sensitive dye. The emission of Ru(bpy)32+ is centered at 610 nm and has a lifetime of 685 ns in argon purged aqueous solution The fiber optic chem. sensor consists of a custom built spectrometer containing argon ion laser light source, detector and associated electronics. A fiber optic cable is employed to guide light into and out of the spectrometer. A known amount of the sensing material in solution is used in a specially designed cell which has a gas permeable Teflon membrane at one end and the other end is coupled with the long cable to the spectrometer.