The effect of oxygen on the electrogenerated chemiluminescence (ECL) of the ruthenium trisbipyridyl-viologen complexes (4,4'-R-2-2,2'-bipyridine)(2)Ru(1-(4-methyl-2,2'-bipyridin-4'-yl-(CH2)(n)-4,4'-bipyridindiium-1'-methyl)(4+) (n = 1, 2, 3, 5, or 8; R = H or Me) has been investigated. This light emission is shown to occur not from a MLCT excited state localized on an unsubstituted bipyridine ligand as had been previously proposed, but from decomposition products formed during the ECL experiments when trace amounts of oxygen are present. These ECL-active species are generated when oxygen reduction products react with the pendant viologen to alter or remove this ligand, thereby disabling the electron-transfer quenching of the excited state by the intact viologen. Electrolysis and extended ECL experiments with methylviologen and the ruthenium-linked viologen compounds verify that a number of products are formed under ECL conditions when oxygen is present in solution. Some of these products have been identified.