Low-frequency dielectric properties of nematic liquid crystal (NLC) cells with and without polyimide alignment layers have been investigated with impedance spectroscopy. It is found that the dielectric behavior in the low frequency regime of NLC cells without polyimide alignment layers can be successfully explained in terms of the diffusive motion of impurity ions. The temperature dependences of the diffusion constant and the concentration of the impurity ions are determined from the dielectric behavior. NLC cells with polyimide alignment layers exhibit the interfacial polarization between the polyimide layers and NLC layer in the low frequency regime. The concentration of the impurity ions dissolved from polyimide layers to NLC layer is estimated.