Thin film capacitors with BaTiO3 as dielectric and RuO2 as electrode material have been prepared by rf-magnetron sputtering at temperatures up to 750 °C and under various oxygen partial pressures. They have been analyzed by complex impedance spectroscopy in the temperature range 20–200 °C. At about 10 kHz, the grain effect is dominant and the electrical and dielectric properties of the bulk grains can be extracted. The activation energies of the conductivity are in the range 30–100 meV, characteristic for hopping of electrons. The dielectric constant at room temperature ranges from 60 to 1500, depending on the preparation conditions. There is a maximum in the temperature dependence of εr that shifts to lower temperatures when the capacitors have been prepared with more oxygen. Above that temperature, Curie–Weiss behavior is observed with characteristic temperatures in the range 250–400 K. Biggest dielectric permittivity and lowest dielectric loss are obtained for films prepared at 750 °C at medium oxygen partial pressures. With increasing oxygen content the low-frequency semicircle and the effective dielectric permittivity, attributed to grain boundaries or an electrode interface layer, increases for the films prepared at all deposition temperatures. The films prepared with high oxygen partial pressure show increased crystallinity and dense columnar structure.