The reaction of the tetrasubstituted benzofuran dioxetanes 2 with various nucleophiles, e.g. Br-, Cl-, I-, and HN(i-Pr)(2), was investigated and the unprecedented bromide ion-catalyzed rearrangement to the hitherto unknown 1-oxaspiro[2.5]octa-5,7-dien-4-ones 4 observed. As the mechanism, an S(N)2 attack of the bromide ion at the O-3 of the peroxide bond is proposed, in which the resulting hypobromite intermediate 8 rearranges to the spiroepoxide 4 by opening of the furan ring and expulsion of the bromide ion through intramolecular nucleophilic attack of the phenolate ion on the O-Br bond. This unique reaction type is limited to the bromide and the iodide ions, since none of the other nucleophiles led to the spiroepoxide 4. On thermolysis, the labile spiroepoxides 4 rearrange to the benzodioxoles 5, and at low temperature, the dimers 6 were formed by the Diels-Alder reaction. 4-Methyl-1,2,4-triazoline-3,5-dione (MTAD) gave with the spiroepoxide 4a the Diels-Alder product 7a. The thermolysis of the dioxetanes 2 gave, besides the conventional C-C cleavage products 3, the dioxoles 5 by rearrangement of the intermediary spiroepoxides 4. In the latter reaction, electron donors favor the formation of the dioxoles 5 and electron accepters the C-C cleavage products 3. The intermediacy of the spiroepoxides 4 in the thermolysis of the dioxetanes 2 was established by trapping of the spiroepoxide 4a with MTAD in the form of the [4 + 2] cycloaddition product 7a. For the thermal rearrangement 2 --> 4, the electron-rich arene moiety is proposed to serve as an intramolecular nucleophile which initiates spiroepoxide formation. Both the bromide ion-catalyzed and the thermal rearrangements are unprecedented in dioxetane chemistry.