The decomposition mechanism and pathway of the firefly dioxetanone have never been explored at a reliable theoretical level before. The difficulty is that the decomposition process includes biradical, charge-transfer (CT) and several nearly degen- erated states, which is in need of multireference method for considering several states simultaneously with an active space available to the whole reaction process. We have investigated the thermolysis of firefly dioxetanone in both neutral (FDOH) and anionic (FDOÀ) forms in gas phase and solvents by the mul- tistate second-order multiconfigurational perturbation (CASPT2) theories in coordination with the ground-state intrinsic reac- exchange-correlation (CAM-B3LYP). The size of selected active spaces have been validated adequate large for handing the whole reaction processes of the decompositions of FDOH and FDOÀ, together with the assistance of state-average (SA) tech- nique. The decomposition processes of FDOH and FDO- were explored in details. The activation energy of FDOÀ is 5.9 kcal/mol smaller than that of FDOH by the CASPT2 cal- culations, which once again indicates the necessary of depro- present calculations, the chemically initiated electron exchange luminescence (CIEEL) and charge-transfer initiated luminescence (CTIL) mechanisms were extremely discussed.