For　years,　dissolved　oxygen　(O-2)　has　been　regarded　as　a　quenching　species　in　Ru(bpy)(3)(2+)　electrochemiluminescent　(ECL)　systems;　however,　in　the　present　study,　O-2　was　found　to　act　as　a　coreactant　for　Ru(bpy)(3)(2+)　ECL　in　Nafion　film,　resulting　in　a　strong　ECL　light　emission.　ECL　experiments　were　carried　out　at　a　Ru(bpy)(3)(2+)/Nafion　film-modified　glassy　carbon　electrode　(GCE)　immersing　in　air-saturated　phosphate　buffer　solution　(pH　7.4).　Scanning　in　the　potential　range　of　+1.5　to　-1.0　V　resulted　in　three　luminescent　processes,　including　two　potential-dependent　luminescence　peaks　(ECL-1　and　ECL-2),　and　one　potential-independent　persistent　luminescence　emission　(CL-P).　Therein,　ECL-2　Occurring　at　potential　less　than　-0.5　V　was　demonstrated　to　be　a　new　chemiluminescent　reactions　between　O-2　and　Ru(bpy)(3)(2+).　In　ECL-2,　Nafion　film　plays　important　roles　in　stabilizing　Ru(bpy)(3)(3+)　and　O-2(center　dot-)　radical　essentially　for　producing　the　new　ECL.　Unlike　previously　reported　ECL　processes,　ECL-2　peak　potential　is　dependent　on　the　reduction　potential　of　the　coreactant　(i.e.,　O-2)　rather　than　the　redox　potentials　of　the　luminopore,　Ru(bpy)(3)(2+),　which　would　provide　a　useful　way　to　probe　O-2,　O-2(center　dot-)　radical,　and　their　stabilities　in　electrochemical　reactions.