A thermal liquid-phase reaction of fullerene C(60) with 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine afforded aza-open-cage fullerene derivative 5 having an eight-membered-ring orifice on the fullerene cage. Compound 5 was found to undergo oxidative ring-enlargement reactions with singlet oxygen under photo-irradiation to give azadioxo-open-cage fullerene derivatives 9 and 10, which have a 12-membered-ring orifice, in addition to a small amount of azadioxa-open-cage fullerene derivative 11, which has a 10-membered-ring orifice. A thermal reaction of 9 with elemental sulfur in the presence of tetrakis(dimethylamino)ethylene resulted in further ring-enlargement to give azadioxothia-open-cage fullerene derivative 15, which has a 13-membered-ring orifice. The structures of 5 and 15 were determined by X-ray crystallography, while those of 9, 10, and 11 were confirmed by the agreement of observed (13)C NMR spectra with those obtained by DFT-GIAO calculations. These reactions were rationalized based on the results of molecular orbital calculations. Following electrochemical measurements, compounds 9 and 10, which have two carbonyl groups on the rim of the orifice, were found to be more readily reduced than C(60) itself (the first reduction potential was found to be 0.2 V lower than that of C(60)), while the first reduction potentials of other open-cage fullerene derivatives, 5, 11, and 15, were nearly the same as that of C(60).