Peroxynitrous acid (ONOOH) was formed by the on-line rapid reaction of acidified hydrogen peroxide with nitrite in a simple flow system. A weak chemiluminescent (CL) signal was observed due to the production of singlet oxygen (O-1(2)) when ONOOH reacted with NaOH, whereas the replacement of NaOH by Na2CO3 markedly enhanced the CL intensity. The predominant CL-enhanced pathway was achieved by the carbonate-catalyzed decomposition of peroxynitrite (ONOO-). Carbonate species was regenerated in the process, that is, carbonate acts as a catalyst. Based on the studies of CL and fluorescence spectra, a possible CL mechanism from the reaction of carbonate with ONOOH was proposed. In brief, ONOOH was an unstable compound in acidic solution and could be quenched into ONOO- in basic media. It was suggested that ONOO-reaction with excess HCO3- proceeded via one-electron transfer to yield bicarbonate ion radicals (HCO3.). The recombination of HCO3. may directly generate excited triplet dimers of two CO2 molecules [(CO2)(2)(*)]. With the decomposition of this unstable intermediate to CO2, the energy was released by CL emission. The addition of uranine into carbonate solution caused enhancement of the CL signal, which was due to a part of excited triplet dimers of two CO2 molecules energy to transfer to uranine, resulting in two CL peaks. (C) 2004 Elsevier B.V. All rights reserved.