Polycrystalline BaTiO3 thin films with thickness ranging from 2100 to 20 000 Å were prepared on platinum substrates using off-axis radio-frequency magnetron sputtering. The variation in room temperature permittivity of the films was investigated with respect to thickness using x-ray diffraction and transmission electron microscopy. All films were ferroelectric and their room temperature permittivity, which was significantly higher than previously reported values, showed a strong dependence on film thickness. Higher permittivity was attributed primarily to the presence of ferroelectric domains. The room temperature permittivity of the thin films showed large variations with grain size, as in the case of BaTiO3 ceramics. The increase in permittivity with increasing film thickness was attributed to the decrease in defect concentration with grain growth. The 20 000 Å film showed an abrupt decrease in permittivity and the presence of an intergranular phase having titanium-excess composition; these phenomena are discussed in terms of domain boundary pinning and recrystallization. © 1997 American Institute of Physics.