The background electrogenerated chemiluminescence (ECL) emission observed only upon electrochemical oxidation of tri-n-propylamine (TPrAH) on a platinum electrode is a limiting factor in ECL analytical techniques and is poorly understood. We studied this reaction in aerated acetonitrile (MeCN) solution with TPrAH oxidized at a constant potential at the Pt surface and observed ECL spectra with an emission band at 630 nm, which is characteristic of the emission of the dimeric (1)Δ(g) state of O(2). No ECL emission was observed when the same solution was deaerated. This background ECL emission is attributed to the reaction between dissolved oxygen and two different products of TPrAH oxidation: the TPrAH(•) radical that reduces O(2) to the superoxide ion and the TPrAH(•+) radical cation that oxidizes this species to singlet O(2).