The lowest-energy lattices of polypyrrole and polythiophene in their planar conformations derived by atomistic lattice simulation agree with the results of diffraction studies on undoped polythiophene. When doped, the structures become layer lattices, the dopant ions occupying intercalation sites. A defect-lattice method to investigate the migration of Cl−and BF−4gives energy barriers 0.5–2.3 eV. The stabilisation of single chains and lattices of helical polythiophene requires olefinic character in the inter-ring bond. The ‘interleaving’ of the strands of adjacent chains requires the dopant ions to occupy sites in the axial region of the helical cavity, along which their migration is expected to be rapid.