The radiationless decay process connected to the breaking of the O-O bridge in benzene endoperoxide has been investigated using high-level ab initio multiconfigurational MS-CASPT2//CASSCF methods. The homolytic rupture of the O-O bond is controlled by a four-state conical intersection. The nine vectors which define the branching space of this high-order degeneracy point have been extracted from two-state conical intersection calculations between the electronic states involved in the degeneracy. The existence of four-state conical intersections indicates that, in general, endoperoxides can deactivate through very efficient radiationless funnels. (c) 2010 Elsevier B. V. All rights reserved.