Direct electrochemiluminescence (ECL) involving oxidized DNA was demonstrated in ultrathin films of cationic polymer [Os(bpy)(2)(PVP)(10)](2+) [PVP = poly(vinyl pyridine)] assembled layer-by-layer with DNA or oligonucleotides. Electrochemically oxidized Os-II sites generated ECL from films containing oxo-guanines on DNA formed by chemical oxidation using Fenton reagent. Films combining DNA, [Ru(bpy)(2)(PVP)(10)](2+), and [Os(bpy)(2)(PVP)(10)](2+) had Os-II sites that produced ECL specific for oxidized DNA, and Ru-II sites gave ECL from reaction with oxo-adenines, chemically damaged DNA, and possibly from cleaved DNA strands.