Chemical interactions between large species are not easy to look at by means of accurate MO calculations and by the usual orbital interaction scheme based on perturbation theory. By transforming the MO’s of the reagent and reactant parts into paired interacting orbitals, we have studied the Diels-Alder reaction of C-60. with butadiene. The interaction involved in this system has been demonstrated to be almost completely localized on a C-C bond at the transition state, bearing a close resemblance to the orbital interactions between dienes and small dienophiles. The addition of butadiene to a C-C bond of C-60 that is common to two annulated six-membered rings has been calculated to have a much lower activation energy than the addition to a C-C bond shared by a six-membered ring and a five-membered ring. The difference in reactivities has been shown to be related qualitatively to the local electron-donating potential and the electron-accepting capacitance of those bonds. The double addition of butadiene has been suggested to be not highly regioselective, both from these local reactivity scales and from the calculated heat of reactions. The possibility of C-60 serving as a diene has also been studied.