The solubility and mode of incorporation of CaO in BaTiO3 were studied by X-ray diffraction, transmission electron microscopy, X-ray microanalysis and by equilibrium electrical conductivity measurements. The solubility limit of CaO in BaTiO3 was determined to be 3 mol% at 1450-degrees-C. The results showed that calcium preferentially occupied titanium sites. CaO inhibits grain growth of BaTiO3 due to the formation of Ba3Ca2Ti2O9 on the grain boundaries of BaTiO3.