The design and performance of a ruggedised dissolved oxygen probe, both in the laboratory and in the field, are reported. This probe is based on phase fluorometric detection of the quenched fluorescence of an oxygen-sensitive ruthenium complex. Dissolved oxygen probes have a number of different applications e.g. in bioreactors, in living systems, in industry and in waste-water treatment plants. The probe reported here has been developed specifically for waste-water monitoring. The oxygen-sensitive complex is entrapped in a porous hydrophobic sol-gel matrix that has been optimised for this application. Light emitting diode (LED) excitation and photodiode detection are employed in a dipstick probe configuration, with the oxygen-sensitive film coated on a disposable Poly(methyl methacrylate)(PMMA) disc, which in turn is designed to optimise guiding of the excitation light into the film. A key element of the design is the common mode rejection of phase between the signal and reference channels, requiring careful selection of the relevant optoelectronic components. Extensive laboratory testing has been carried out on this sensor, and it exhibited high performance with limit of detection typically 6 ppb, sensor resolution of 15 ppb at 9 ppm oxygen concentration, and excellent long term stability of 0.1 ppm per week. A first phase of field testing at a UK waste-water treatment plant has taken place and a second is planned for 2002. Also reported here is the optimisation of films, which have Platinum Octoethylporphyrin keytone (PtOEPk) entrapped as the oxygen-sensitive complex. The fluorescence from the PtOEPk is quenched in the presence of oxygen, in a similar way to the ruthenium complex. The longer lifetime of PtOEPk allows for greater sensitivity at lower oxygen concentrations. Several different support matrices for the porphyrin have been investigated using a fibre optic sensor, optimised for working with PtOEPk.