Resistance and capacitance measurements, as a function of frequency, were carried out to analyze and model the electrical behavior of undoped titanium dioxide (TiO2) films. The sensing measurements were carried out in a chamber with independent temperature, pressure, gas composition, and flow rate control. With the aim of evaluating the electrical response of the sample as a function of the surrounding atmosphere, a frequency range of 100 Hz–4 MHz was used (vacuum or air at atmospheric pressure). The resistance and capacitance responses were obtained with the help of impedance spectroscopy technique. The impedance response versus frequency allows the discrimination between grain-boundary capacitance (Cgb) and resistance (Rgb) contributions. A model, taking into account the possible effects due to grain size is proposed in order to work out the permittivity of the material.