The solubility and mode of incorporation of CaO in CaTiO₃ were studied by X-ray powder diffraction, scanning and transmission electron microscopy, electron probe microanalysis and equilibrium electrical conductivity measurements. The presence of Ca₄Ti₃O₁₀ in samples containing > 0.3 mol% excess CaO was confirmed by direct microscopic examination. Measurements of the equilibrium electrical conductivity showed no detectable shift in the conductivity profile for CaO-excess CaTiO₃, thus setting an upper limit of 100 p.p.m. for the solubility of CaO in CaTiO₃. The excess CaO is incorporated into CaTiO₃ by formation of Ruddlesden-Popper type planar faults, which are layers of CaO coherently intergrown with CaTiO₃. Ordering of these planar faults leads to the formation of the stable compound Ca₄Ti₃O₁₀.