This paper describes how the DDCI method can be applied to study the reactivity of dioxetane, a molecule that decomposes thermally into 2 formaldehyde molecules. One of these molecules is in an excited state that decays radiatively. Previous experimental and theoretical studies proposed that this decomposition takes place through a stepwise mechanism, but the relative energies of the two transition states involved in the reaction have been a source of controversy. While experimental evidence shows that the first transition state is higher in energy, one of the latest and most accurate theoretical studies does not produce the same results. In this paper the projection of the reaction path over the ground-state potential energy surface (PES) is calculated with a conventional method. We calculate the energies of the species on the excited states involved in the reaction by adding the energy of the vertical transition to the ground-state energies. To do so we use the DDCI method, which was specifically designed to calculate energy differences. The first transition state, located in the ground-state PIES, is found to be higher in energy than the second one, located in the T, PES. This result agreed with the experimental result.