Oxygen quenching of long lifetime fluorescent dyes takes place when the excited state of the fluorescent dye decays non-radiatively to the ground state by energy transfer to the oxygen. Thus, by monitoring the intensity of fluorescence, the oxygen dependence is seen as a decrease in intensity for an increase in oxygen concentration. The Ruthenium based fluorescent dyes used m this work have lifetimes of the order of 0 7ms which allows them to be oxygen sensitive, and two ^ I i I such dyes, Ru (bpy)3 and Ru (phen)3 , were extensively tested for use in a fibre optic oxygen sensor. The oxygen sensitive dyes were incorporated into a fibre optic system which was used to transmit the 488nm light from an Ar ion laser to the sensor tip and also to carry the fluorescence signal back to the detector. The fibres from a 4 - port coupler were used m this sensor as follows 1) for the launching of excitation hght, 2) for housing the sensmg probe tip, 3) for the return of fluorescence signal to the detector and 4) for monitoring the stability of the Ar ion excitation source. The oxygen sensing dye is immobilised in Nafion ion exchange resin with which a cavity etched mto the fibre end was coated. A method was developed for securely attaching the Nafion polymer to the fibre surface. The final probe was found to respond to changes in dissolved oxygen partial pressure of 0 053 bar, while the spatial resolution was estimated at 5Qwm which corresponds to the core diameter of the fibre used m the work. The sensor was found to operate equally well m both aqueous and gaseous environments without requiring separate manufacturing procedures. The etching of graded index glass optical fibres in conc HF acid was also investigated. The resultant cavity was found to develop in a manner not predicted by the current etching models. An etching model was developed to account for the development of the etched cavity which was observed using both optical and electron microscope techniques. The etching process was also modelled on a computer.