Abstract Four Aurivillius type systems with generic formula Bi 4− x R x Ti 3O 12, where R x =Pr, Nd, Gd, and Dy, have been synthesised to investigate the physical effects of R x on the host lattice Bi 4Ti 3O 12. Particularly we report here the crystallographic characteristics, as measured by X-ray, and the dielectric properties of the resulting compounds. The layered orthorhombic structures show variations of the a/b distortion comparable with that of Bi 4Ti 3O 12. Impedance spectroscopy is used to display the electrical response over a wide temperature range, from room temperature up to 850 °C. The conductivity diminishes as the size of R x grows; activation energies are reported. The rare earth concentration determines the ferroelectric characteristics of the compounds: for small x, they behave as typical ferroelectrics, but all of them tend to become relaxors when x grows. The structure seems to favour the appearance of ferroelectricity when the atomic size of the rare earth diminishes. SEM reveals microscopic composition fluctuations in all samples. Using the specimens as if they were the detectors of the electron microscope, we have determined the type of charge carrier involved in the conductivity phenomena.