Abstract Lucigenin (10,10′-dimethyl-9,9′-biacridinium dinitrate, DBA2+2NO3−) was found to be a CT complex between DBA2+ and NO3−. In the reaction of lucigenin with several nucleophiles in the absence of molecular oxygen or oxidizing agents the following four types of products were obtained, depending upon the relative power of the nucleophiles to donate electrons to DBA2+: (1) salts of DBA2+ (DBA2+2X−) which are the CT complex between DBA2+ and the nucleophiles, such as Cl−, Br−, SCN−, and I−, (2) the cation radical DBA\underset.+ produced by one-electron transfer from nucleophiles, such as C6H5COCH2− and CH3COCH2−, to DBA2+, (3) the biradical DBA\underset.2 produced by two-electron transfer from nucleophiles such as OH−, CN−, CCl3−, and C6H5S− and (4) 10,10′-dimethyl-9,9′-bi(dihydroacridinylidene) produced from DBA\underset.2. Only the nucleophiles of (3), which produced DBA\underset.2 brought about the luminescence of lucigenin in the presence of oxygen, and DBA\underset.2 also showed luminescence in organic solvents in the presence of oxygen. From these findings it was concluded that the first process of the chemiluminescent reaction of lucigenin was the electron transfer reduction of DBA2+ by the nucleophiles to form the biradical DBA\underset.2.