The layered intergrowth phases Bi4MO8Cl (M = Nb, Ta), structurally related to the well-known Aurivillius phase ferroelectrics, have been studied using SHG and a.c. impedance techniques. Both undergo a polar to centrosymmetric phase ferroelectric-type transition (Tc = 765 and 640 K for M = Nb and Ta, respectively). The crystal structure of the low temperature phase of Bi4NbO8Cl has been re-analysed, and a tentative structure for Bi4TaO8Cl determined, from powder neutron diffraction data. The basic structural features are confirmed, but significant details differ from the previously reported X-ray structure of Bi4NbO8Cl.