The decomposition reaction of dimethyl-1,2-dioxetanone in dichloromethane was studied by using a DFT approach. The low efficiency of triplet and singlet excited-state formation was rationalised. A charge-transfer process was demonstrated to be involved in the chemiluminescence process. Present and previous results allow us to define an interstate crossing-induced chemiexcitation (ICIC) mechanism for the chemiluminescence of dioxetanones. Charge transfer is needed to reach a transition state, in the vicinity of which direct population of excited states is possible. The chemiexcitation process is then governed by singlet/triplet intersystem crossings. Structural modifications then modify the rate of these crossings and the singlet ground and excited-state interaction, thereby modulating the efficiency of this process and the spin of the resulting products.