Efficient and stable quenching of electrochemiluminescence ( ECL) of tris( 2,2'-bipyridine)-ruthenium( II) by oxidizing ferrocene methanol ( FcMeOH) at the electrode is reported. Bimolecular energy or electron transfer between Ru( bpy)(3)(2+*) and ferrocenium ( Fc(+)), the oxidized species of Fc, along with suppression of radical reactions is suggested as the mechanism for quenching ECL. Fc shows more efficient quenching of ECL compared with the known quenchers phenol and 1,1-dimethyl-4,4'-bipyridine dication ( MV2+). The ECL quenching rate constant was 5.6 x 10(10) M-1 s(-1). Using Fc as a quencher label on a complementary DNA sequence, an intramolecular ECL quenching in hybridized oligonucleotide strands has been realized. With essentially complete quenching efficiency, this system has the potential for application to sequence-specific DNA detection.