The sintering behaviour of BaTiO 3 powders and the resulting microstructure of the corresponding ceramics are controlled by the stoichiometry of the outer region of the grains of the starting powder. These grains exhibit a core-shell structure due to the water attack during powder milling. The hydrolysis results in the formation of lattice-hydroxyl groups. The existence of these processing-related hydroxyl defects is detected by 1 H magic-angle spinning nuclear magnetic resonance ( 1 H-MAS NMR) as well as differential thermal analysis/thermal gravimetry (DTA/TG) measurements and also verified by electron energy-loss spectroscopy (EELS) in combination with high-resolution transmission electron microscopy (HRTEM). The results are confirmed by detecting the near-edge structure of the O-K edge, the details of which can be interpreted by quantum-mechanical calculations of the electronic density of states in the conduction band.