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The characteristics of two distinct perylene chemiluminescence (CL) decay curves generated by the reaction between bis(2,4-dinitrophenyl) oxalate (DNPO) and H 2 O 2 in the absence of base catalysts were investigated. When the intensity of peroxyoxalate chemiluminescence (PO-CL) vs. time was measured under relatively low H 2 O 2 and high DNPO concentrations in ethyl acetate, a slowly decaying curve appeared. However, upon increasing the H 2 O 2 concentration under the same DNPO-CL reaction conditions, two distinct emission maxima were observed: a fast decaying CL curve and a slowly decaying CL curve. The fast decaying CL curve appeared alone when the H 2 O 2 concentration greatly exceeded the DNPO concentration in ethyl acetate. To learn more about the properties of both CL decay curves, we further investigated the effects of adding H 2 O or phenols having different nucleophilicity [2,4-dinitrophenol (DNP), 2,4-dichlorophenol (DCP), pentachlorophenol (PCP), and 2,4,6-trichlorophenol (TCP)] and the stability of DNPO used in the CL reaction. The slowly decaying curve was predominant with the addition of H 2 O and relatively strong nucleophile. Based on the observed results, we propose that the likely high-energy intermediates generated in DNPO-CL reactions without added base catalysts are hydroperoxyoxalate ester and a six-membered cyclic peroxide.