A dissolved oxygen sensor for marine application is reported which is based on a ratiometric approach to the fluorescence intensity quenching of the ruthenium dye [Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline)](2+). Both the ruthenium complex and the coumarin derivative dye have been entrapped in a porous sol-gel film. The membrane shows good optical response on exposure to low levels of dissolved oxygen in seawater. The design and limitations of such an optode membrane are discussed.