Phenalenone (PN) is a very efficient singlet oxygen sensitiser in a wide range of solvents. This work uses ab initio quantum chemical calculations (CASSCF/CASPT2 protocol) to study the mechanism for populating the triplet state of PN responsible for this reaction, the (3)(π-π*) state. To describe in detail this reaction path, the singlet and triplet low-lying excited states of PN have been studied, the critical points of the potential energy surfaces corresponding to these states located and the vertical and adiabatic energies calculated. Our results show that, after the initial population of the S(2) excited state of (π-π*) character, the system undergoes an internal conversion to the (1)(n-π*) state. After populating the dark S(1) state, the system relaxes to the (1)(n-π*) minimum, but rapidly populates the triplet manifold through a very efficient intersystem crossing to the (3)(π-π*) state. Although the population of the minimum of this triplet state is strongly favoured, a conical intersection with the (3)(n-π*) surface opens an internal conversion channel to this state, a path accessible only at high temperatures. Radiationless deactivation processes are ruled out on the basis of the high-energy barriers found for the crossings between the excited states and the ground state. Our computational results satisfactorily explain the experimental findings and are in very good agreement with the experimental data available. In the case of the frequency of fluorescence, this is the first time that these data have been theoretically predicted in good agreement with the experimental results.