Abstract In this paper we report preliminary results from an intrinsic fibre-optic oxygen based on phase fluorimetry. Phase fluorimetry is a method of measuring the luminescence lifetime of a fluorophore and, where suitable, has many advantages over other reported optical sensing techniques such as absorption and fluoresecnece intensity monitoring. Lifetime measurements are absolute quantities, which offer the possibility of inherent referencing, and are usually independent of indicator concentration, photobleaching the excitation source intensity. The long excited-state lifetime of ruthenium polypyridyl complexes are efficiently quenched by oxygen. Sol-gel immobilization of the ruthenium complex (Ru II-tri(4,7-diphenyl-1,10-phenanthroline)) on a multimode optical fibre yields highly durable, inert, microporous claddings which exhibit almost complete quenching under evanescent-wave excitation. The fluorophore decay curve exhibits a double exponential behaviour consisting of a fast and a slow component, both of which undergo quenching on exposure to oxygen. These data are used to model and expected behaviour of the sensor in phase fluorimetric mode. The predictions are in close agreement with experimental measurements performed with the coated fibre under blue light-emitting diode (LED) excitation. An optimum modulation frequency is identified and the performance of the intrinsic oxygen sensor in real-time measurement mode is reported. The use of an inexpensive light source, combined with a simple fabrication technique and the advantages of the phase fluorimetric method, facilitates the production of low cost, high performance optical oxygen sensors.