Reaction of 1,2-dimethyl-1,2-dihydrophthalic anhydride (8) with t-BuOONa at 40 °C gave o-xylene as the only observed product. This reaction is proposed to occur through intermediacy of the anionic peroxide 10 whose conversion to o-xylene is calculated to be exothermic by 84 kcal/mol. Extension of this reaction to systems that do not have the possibility for gaining aromatic stabilization was unsuccessful.