The effect of the surface preparation in samarium doped semiconducting barium titanate [(Ba1-xSmx)TiO3] ceramics with (Ba, Sm)/Ti ratio of 1.000 was studied by means of isotope tracer technique using a secondary ion mass spectrometer. The surfaces of specimens were prepared with the chemical mechanical polishing (CMP) with colloidal silica slurry or the mechanical polishing (MP) with diamond paste. The oxygen diffusion coefficients obtained in the CMP samples were small compared to those in the mechanical polished samples. This fact suggests that the surface prepared with CMP has less oxygen defect concentration. Moreover, it was also indicated that high temperature treatment over 1000 °C is required for annihilation of defects formed by MP. The oxygen diffusion study used CMP sample brings the useful information on the oxygen defect chemistry in Sm doped BaTiO3.