An open-cage C-70 fullerene with a 13-membered ring-opening and a bis(hemiacetal) moiety was synthesized by the reaction of the corresponding opencage C-70 diketone with nucleophilic oxidizing agents. The size of the cage opening could be expanded by a subsequent dehydration reaction. Reaction of the thus obtained open-cage C-70 tetraketone with elemental sulfur in the presence of tetrakis(dimethylamino)ethylene resulted in the formation of the first example of an open-cage C69S thiafullerene with a 12-membered ring-opening. The formation of this sulfur-containing heterofullerene reflects a significantly different chemical reactivity for the open-cage C-70 tetraketone relative to its C-60 analogue. The structures of all novel compounds were unambiguously determined by single crystal X-ray diffraction analyses, in addition to which the electrochemical properties of the thiafullerene C69S were examined and compared with those of the corresponding C-70 analogue.