Differing from its diphenyl analog,2,5-dibromo-1,6-dimesityl hexandione-[1,6],formed neither cyclopentane derivatives nor pyridinium salts,dehydrobromination product,1,6-dimesityl hexadiene-2,4-dione-[1,6] but the when it was treated with pyridine at about 140°.1,6-Dimesityl hexadiene-2,4-dione-[1,6] so obtained was golden yellow needles,melting at 218-219°.It contained no bromine and decolorized both alkaline permanganate and bromine solutions.Its molecular weight (338)was found close to the theoretical value (346) by Rast camphor method.It gave 1,6-dimesityl hexanedione-[1,6] by catalytic hydrogenation and 1,6-dimesityl 2,3,4,5-tetrabromohexanedione-[1,6] by addition of bromine.Its structure was finally proved by synthesis from muconyl chloride and mesitylene by Friedel-Crafts reaction.