A Nafion(R)/ruthenium-oxide pyrochlore (Pb2Ru2-xPbxO7-y) chemically modified electrode (CME) was constructed for the determination of dissolved oxygen (DO). Nafion(R), a perfluorosulfonated ion-exchange polymer, was first spin coated onto a glassy carbon electrode. Ruthenium oxide pyrochlore particles were then synthesized in the Nafion(R) matrix by treatment of Ru3+, Pb2+-exchanged polymer in alkaline aqueous solution with purging of O2 at 53-degrees-C for at least 1 day. The catalytic reduction of DO on a Nafion(R)/ruthenium-oxide pyrochlore CME results in fast and sensitive determination of DO in both acid and alkaline environments. The optimal conditions for the construction of such a CME were thoroughly studied. The mechanism of detection is also discussed. The analytical utility of the CME is demonstrated by application to underground water.