The photocycloaddition of cyclic enones to C-60 is a general reaction. The addition was initially explored using a crown ether-tagged C-60 molecule which allows convenient monitoring of the reaction course by electrospray ionization mass spectroscopy (ESI-MS) after complexation with metal ions, usually K+. A Hanovia 450-W lamp was used to excite a benzene solution of fullerene and enone, with the latter in large excess. A series of experiments in which the concentration of reagents and reaction time were systematically varied established that adduct formation occurs only upon light absorption by the enones. Thus, excitation at 532 nm or in the UV range using low enone : C-60 ratios did not lead to adduct formation. ESI-MS analysis showed that monoaddition was favored at short irradiation times, and that up to seven enone units could be added to a single fullerene molecule upon longer exposure to light. : C-60 ratios did not lead to adduct formation. ESI-MS analysis showed that monoaddition was favored at short irradiation times, and that up to seven enone units could be added to a single fullerene molecule upon longer exposure to light. : C-60 ratios did not lead to adduct formation. ESI-MS analysis showed that monoaddition was favored at short irradiation times, and that up to seven enone units could be added to a single fullerene molecule upon longer exposure to light.