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.