2-Bromo-2, 3-dimethylbutane decomposes when treated with phenylmagnesium bromide and cobaltous bromide to give 2, 3-dimethylbutanc (35%), 2, 3-dimethylbutene-1 (31%) and 2, 3 dimethylbutene-2 (ca. 2%). l-Chloro-l-methylcyclohexane decomposes when treated with methylmagnesium bromide or isopropylmagnesium bromide and cobaltous bromide to give mainly a mixture of methylcyclo-hexane, l-methylcyclohexene and methylenecyclohexane. A small amount of a high-boiling product, probably a dimer, was also obtained. In the unsaturated monomeric fraction considerable amount of methylenecyclohexane was found.