Barium titanate (BaTiO3) base-metal electrode multilayer ceramic capacitors of an X7R-formulation, sintered at 1200 degrees C under low oxygen partial pressures (of pO(2)approximate to 10(-9) and 10(-11) atm, respectively), followed by annealing at 1000 degrees C in an atmosphere containing a higher oxygen partial pressure (of pO(2)approximate to 10(-5)-10(-6) atm), have been analyzed for crystalline phases using X-ray diffractometry, for microstructure using transmission electron microscopy, and for microchemistry using energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy. The classical core-shell structure characterized by a core consisting of tetragonal ferroelectric {011} domains and featureless shell (designated type I) was observed only in sample A sintered in pO(2)approximate to 10(-9) atm. For sample B sintered in pO(2)approximate to 10(-11) atm, the core-shell structure is predominantly type II, consisting of a featureless shell similar to type I, but a core of modulated domains. The core of type II contained incommensurately modulated {111} superlattice domains along < 111 >. The superlattice can be described by a displacive modulation with incommensurate wave vectors k(1)=0.58a(*), k(2)=0.58b(*), and k(3)=0.58c(*). It is due to the ordering of defect associates (Ti'(Ti),V-O)(center dot) generated extrinsically from sintering in low pO(2). Shell thickness was determined by the lattice diffusion of Ca2+ solute cations into BaTiO3 grains during sintering. The core-shell interface became less distinguishable in type II because defect associates (Ti'(Ti),V-O)(center dot), unlike those in type I, were not completely eliminated by re-oxidizing in pO(2)approximate to 10(-5)-10(-6) atm, but became ordered along < 111 > and gave rise to structural modulation.