The pseudo-binary system of BaTiO3-SrTiO3 ceramics offering potential applications in the electronic industry, particularly for the passive components, has been studied for its diffuse phase transition over the temperature range of +150oC and -50oC. This research concentrating on the interdiffusion between two sintered layers of such perovskite is a continuation of study, conducted by this author¡¦s group over the past years. Two-layer BaTiO3-SrTiO3 stacks were sintered at 1300oC and annealed for various time periods to investigate if and how the interdiffusion occurs across the BaTiO3-SrTiO3 interface. Optical microscopy reveals an interface layer consisting of polytitanate second phases, which appear to be large, chunky grains initially. Both results obtained from X-ray diffractometry and micro-chemical analysis using the energy-dispersive spectrometry, equipped with the scanning electron microscopy, suggest that the second phases are: Ba4Ti13O30, Ba2Ti9O20, Ba6Ti17O40 and BaTi2O5. These polytitanates are produced from the solid-state reaction between BaTiO3 and TiO2, which is left behind in the BaTiO3 layer when Ba2+ being the faster diffusion A-site cation have moved across into the SrTiO3 layer in a significantly higher content. The interface phases grow progressively to a coherent second-phase layer upon prolonged annealing for 100 h. It is revealed by the transmission electron microscopy that residual pores, similar to the Kirkendall type in the classical Cu-Zn diffusion couple, generated at ~100 £gm away from the interface and located in the BaTiO3 layer. This is attributed to the significantly different lattice diffusivities between two A-cations, i.e. Ba2+ being faster than Sr2+ by approximately three times, with A-site vacancies ( ) created in the grains of the BaTiO3 layer. Together with B-site cation vacancy ( ) and oxygen vacancy ( ), similar to the prismatic loops formed in quenched aluminium, condensation of vacancies via a reverse Schottky defect reaction has formed such Kirkendall-like pores within BaTiO3 grains. Interdiffusion has resulted in forming the solid solutions of (Ba,Sr)TiO3, with Sr2+ being solute cation, and (Sr,Ba)TiO3, with Ba2+ being solute cation, in the initial layers, respectively, and the characteristic core-shell grains responsible for the diffuse-phase transition. A mechanism of how cation diffusion produces the core-shell grains in both layers, modified from Bow (1990) and Liu (1991), is proposed.