The structure and chemistry of planar defects in Nb2O5- and Bi2O3-doped BaTiO3 ceramics that exhibit "core-shell" microstructures have been examined using a combination of conventional transmission (CTEM) and high-resolution (HREM) and scanning transmission (STEM) electron microscopy and microanalysis. In addition to ferroelectric domain boundaries within the core, twins and "stacking faults" were observed, both with interface planes lying along {1 1 1}. Unlike the twins, stacking faults were observed only within the paraelectric shell region of the BaTiO3 grain; these planar defects are enriched in Nb and Bi relative to the surrounding matrix. The combined CTEM contrast analysis and HREM observations suggest a fault displacement vector normal to the fault plane of magnitude 1/2 < 1 1 1 > + x < 1 1 1 >(x = 0.1); a proposed fault structure is based on a double BiO33- layer with bismuth cations occupying barium sites and charge compensation by Nb5+ substitution in adjacent octahedral sites. In addition, the incorporation mechanism of Nb2O5 and Bi2O3 into BaTiO3 is discussed with respect to microanalysis results.