Co-evaporation of solutions of C60F18 and tetrathiafulvalene in toluene produces an unsymmetrical C60F16:tetrathiafulvalene adduct through a unique six-electron cycloaddition involving displacement of two fluorine atoms by a terminal CC double bond of the fulvalene.T he adduct rearranges into two further adducts, one of which is characterised as a new type of fullerene derivative, a thiiranofullerene, formed by elimination of a thioketene moiety from the tetrathiafulvalene adduct.T he initial addition also produces a bisadduct in which the addends comprise one tetrathiafulvalene molecule and one in which carbon disulfide has been eliminated. T he latter adduct involves cycloaddition of an unsaturated aromatic dithiolactone moiety.