High resistivity diffusion layers are formed by reoxidation between 380 and 750 °C in reduced BaTiO3 ceramic. The potential distribution is measured with an applied voltage at these samples. In addition, observations with the polarizing microscope, resistance and gravimetric measurements are made. The results show that the diffusion of oxygen vacancies is hindered by grain boundaries. The quantities controlling the diffusion mechanism, such as the grain boundary diffusion velocity and the effective diffusion coefficient at longer diffusion distances, are determined experimentally.