The 2-methylallyl cation has been generated in liquid sulphur dioxide from 2-methylallyl iodide and silver trichloroacetate in the presence of cyclopentadiene, cyclohexadiene, and furan. Some new bridged seven-membered ring olefins, 3-methylbicyclo[3,2,1]octa-2,6-diene, 3-methylenebicyclo[3,2,1]oct-6-ene, 3-methylbicyclo-[3,2,2]nona-2,6-diene, have been isolated, and it is suggested that they arise from a concerted cycloaddition of the allylic cation to the conjugated diene.