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–gel-derived silica films were fabricated by dip-coating on to planar and optical fibre substrates. The films were pre-doped with the oxygen-sensitive ruthenium complex [R^II–tris(4,7-diphenyl-1,10-phenanthroline)], the fluorescence of which is quenched in the presence of oxygen. The structure and behaviour of sol–gel films are dependent on the fabrication parameters. 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 increasing the ratio of modified precursor, the quenching response in the aqueous phase increases. A very low limit of detection, 6 ppb, was determined for the modified films. Using a high-brightness blue LED, combined with a miniature photodiode detection system, these results indicate the potential for a low-cost, high-performance, portable dissolved oxygen sensor for use in many varied situations from aeration control to on-line river pollution monitoring.