This letter presents a comprehensive impedance spectroscopy characterisation of Magneli phases (TinO2n-1) over a range of temperatures, which are of interest in electrochemistry and sensing applications, with the aim to enhance the understanding of their electrical properties and influence their microstructure. The impedance of the TinO2n-1 can be resolved into two different contributions, namely the grain bulk (RB) and grain boundaries (RGB). The ac conductivity increases with frequency and temperature, following a universal power law. The high relative permittivity (105-106), which is relatively frequency independent from 0.1Hz to 100kHz, is attributed to the presence of insulating grain boundaries (RGBRB) creating an Internal Barrier Layer Capacitor (IBLC) effect. Above 100kHz, the grain boundaries begin to contribute to the ac conductivity and the permittivity drops sharply.