2-Bromo-2-fluoro-1-alkylidenecyclopropanes heated in polar solvents undergo isomerization into substituted 3-bromo-2-fluoro-1,3-butadienes. 2-Bromo-3-methyl-1-methylene-3-phenyl-2-fluorocyclopropane at heating afforded a mixture of substituted 1-(bromofluoromethylene)cyclopropane and isomeric 1-bromo-1-fluoro-1,3-butadienes.