Cycloaddition reactions of buckminsterfullerene (C-60) with mesoionic compounds gave the 1 to 1 cycloadducts reacted at the 6-6 double bond of C-60 as a sole product. A thermal reaction of the adducts proceeded in a reversed fashion and another dienophile trapped the liberated mesoionic compounds. Thermogravimetric analyses of the adducts reveal that extrusion of small fragment occurs at controlled temperature. UV-VIS spectra of the adducts altered with isosbestic points upon irradiation of them. Preparative photoreaction gave only a black compound insoluble in major organic solvents.