The grain boundary space charge depletion layer in acceptor-doped SrTiO3 and BaTiO3 ceramics is investigated by an impedance analysis in the time domain and is described in terms of a simplified back-to-back double Schottky barrier model. A numerical simulation is employed to analyze the defect chemistry in the depletion layer and to elaborate a refined Schottky model. The local distribution of the donor-type grain boundary states causing the depletion layer and the resulting band bending are discussed.