TEM and XRD have been used to characterise the antiferroelectric-ferroelectric phase boundary which exists in the (Pb1-xBax)(Zr1-xTix)O-3 solid solution close to the PbZrO3 end member. Electron diffraction patterns and XRD spectra from the antiferroelectric phase could be indexed according to the PbZrO3 structure with superlattice reflections typically present at the +/-1/4{hk0} pseudocubic positions. By XRD, the polar ferroelectric phase appealed to be rhombohedral but electron diffraction patterns showed the presence of superlattice reflections at the 1/2{hk0} positions, indicating that the unit cell size was doubled with respect to the simple perovskite lattice. The intensity of these reflections was observed to decrease with increasing Ba and Ti concentration. A combination of dark field and high resolution dark field images showed the presence of antiphase boundaries associated with these reflections which were highly mobile during il radiation in the electron beam. (C) 1997 The Institute of Materials.