The reactive sites in [4+2] cycloadditions to C-60 are discussed based on vibronic coupling density (VCD) for the reaction mode. The frontier orbital theory encounters difficulty to predict reactive sites for such a large system, because of the delocalization of the frontier orbitals. The VCD analysis clearly illustrates the existence of ethylene moieties embedded in the C-60 cage which behaves as a dienophile. The C-60 cage contains six isolated double bonds as a functional group. The VCD for the reaction mode can be regarded as a reactivity index. (C) 2012 Elsevier B.V. All rights reserved.