2,3-Dihydro-2,3-bis-(methylene)thiophene and its benzo derivative, generated flash-pyrolytically, polymerise or are trapped with HCl and PhSH but only the latter gives Diels-Alder adducts; electrocyclisation of the 3-phenylmethylene derivative is accompanied by skeletal rearrangement to give naphtho[1,2-b]thiophene.