A dissolved oxygen sensor based on the fluorescence intensity quenching of a ruthenium dye complex, [Ru(II)-tris(4,7-diphenyl-l,10-phenanthroline)]2+, which has been entrapped in a porous sol-gel film, is reported. The aim of the project was to develop and test a portable instrument (prototype), based on a laboratory-developed sensor technique, for use in marine and inland seas. Issues for optimum sensor performance included stability and reproducibility of the prototype sensor measurements in deionised and marine water. Investigations to optimise the sensing films, such as leaching, thickness and coating of an additional optical barrier layer, are discussed. Measurements with the prototype and a laboratory test system are compared. Problems with temperature drift and slide insertion reproducibility were encountered. These effects were fully characterised and design improvements are suggested. Finally alternative approaches to intensity-based dissolved oxygen sensing are discussed.