The electronic excited states of anthracene-9,10-endoperoxide giving rise to singlet oxygen generation (102) or homolytic O-O cleavage have been unambiguously characterized using multi-configurational second order perturbation theory MS-CASPT2/CASSCF, with inclusion of solvent interactions, femtosecond polarization resolved UV/IR pump-probe, and steady-state fluorescence experiments. The S, state, responsible for homolytic O-O cleavage, is observed at 34400 cm(-1). Determination of the transition dipole moment direction of the 37000 cm(-1) band, confirms the assignment of the S-4 state, from which O-1(2) is generated, and allows assigning the other electronic states. Furthermore, it is demonstrated that APO can emit after electronic excitation. (c) 2007 Elsevier B.V. All rights reserved.