A dissolved oxygen sensor based on the quenching of fluorescence from a ruthenium dye complex entrapped in a porous sol-gel film is reported. Sol-gelderived silica films were fabricated by dip-coating onto planar and optical fibre substrates. The films were pre-doped with oxygen-sensitive ruthenium complex, [Ru(n)-tris(4, 7-diphenyl-1,10-phenanthroline)]2+, whose fluorescence is quenched in the presence of oxygen. The structure and behaviour of sol-gel films is dependent on the fabrication parameters. By varying these parameters, different microstructures and consequently different quenching responses to dissolved oxygen are obtained. In particular, enhancement of the surface hydrophobicity increases the quenching response in water. This is achieved by using suitable proportions of modified precursors of silica of the form R(OEt)3Si, where R is an alkyl group, in the standard fabrication procedure. It is shown that by extending the aliphatic group of the modified precursors, the quenching response in aqueous phase increases. In this way, the film has been optimised for dissolved oxygen sensing. The development of a dissolved oxygen sensor is mapped from the preliminary oxygen/dissolved oxygen measurements using a laser-based characterisation system through to a small, portable, all-solid state system, which can be used in different modes e.g. fluorescence intensity and phase fluorimetric modes, as well as two different configurations, planar- and fibre-based configurations, all of which have been demonstrated here.